Highly porous and mechanically strong ceramic oxide aerogels

ABSTRACT

Structurally stable and mechanically strong ceramic oxide aerogels are provided. The aerogels are cross-linked via organic polymer chains that are attached to and extend from surface-bound functional groups provided or present over the internal surfaces of a mesoporous ceramic oxide particle network via appropriate chemical reactions. The functional groups can be hydroxyl groups, which are native to ceramic oxides, or they can be non-hydroxyl functional groups that can be decorated over the internal surfaces of the ceramic oxide network. Methods of preparing such mechanically strong ceramic oxide aerogels also are provided.

This application is a divisional of U.S. utility patent application Ser.No. 11/266,025 filed Nov. 3, 2005, which application claims the benefitof U.S. provisional patent application Ser. No. 60/624,666 filed Nov. 3,2004. The contents of both of these applications are incorporated hereinby reference.

STATEMENT OF GOVERNMENT-SPONSORED RESEARCH

This invention was made with U.S. government support under contract Nos.NCC3-1089 and NCC3-887, both NASA Grant/Cooperative Agreements awardedby the National Aeronautics and Space Administration. The U.S.government has certain rights in this invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Ceramic aerogels are among the most highly porous and lowest densitymaterials. Their high porosity means that 95% or greater of the totalbulk volume of a ceramic aerogel is occupied by empty space (or air),producing excellent thermal as well as sound insulating qualities. Inaddition, their high specific surface area (e.g. on the order of600-1000 m²/g) should make them well suited for numerous applications,including as adsorbent beds for chemical separations, as catalystsupports, as platforms for solid state sensors, etc. Unfortunately,conventional ceramic aerogels are physically and hydrolytically veryunstable and brittle. Their macro-structure can be completely destroyedby very minor mechanical loads or vibrations, or by exposure tomoisture. In addition, over time, these materials tend to produce fineparticles (dusting) even under no load. Consequently, there has beenlittle interest in ceramic aerogels for the above-mentioned as well asother applications, despite their excellent properties, simply becausethey are not strong enough to withstand even minor or incidentalmechanical stresses likely to be experienced in practical applications.To date, such aerogels have been used almost exclusively in applicationswhere they will experience no or almost no mechanical loading.

2. Description of Related Art

U.S. Patent Application Publication No. 2004/0132846, the contents ofwhich are incorporated herein by reference, describes an improvementwherein a diisocyanate is reacted with the hydroxyl groups prevalent onthe surfaces of secondary (φ 5-10 nm) particles of a silica aerogel toprovide a carbamate (urethane) linkage. Additional diisocyanate monomersare further polymerized to produce a network of polyurea chains betweenthe carbamate linkages of respective pairs of hydroxyl groups present onthe secondary particles, resulting in a conformal polyurea/polyurethanecoating over the silica backbone. The resulting structure was found tohave only modestly greater density than the native silica gel (2-3 timesgreater), but more than two orders of magnitude greater mechanicalstrength, measured as the ultimate strength at break for comparablydimensioned monoliths. The present invention provides furtherimprovements beyond what is disclosed in the 2004/0132846 publication.

SUMMARY OF THE INVENTION

A structure is provided, having a solid-phase three-dimensional networkof ceramic oxide particles. The particles have non-hydroxyl functionalgroups bound to surfaces thereof, and the network of ceramic oxideparticles is cross-linked via organic polymer chains that are attachedto the particles via reaction with at least a portion of theirsurface-bound non-hydroxyl functional groups.

A method of preparing a polymer cross-linked ceramic oxide network alsois provided. The method includes the steps of preparing a solid-phase,three-dimensional ceramic oxide network that is mesoporous and hasnon-hydroxyl functionality provided over internal surfaces of thenetwork, and reacting at least a portion of non-hydroxyl functionalgroups attached to those internal surfaces with an organic polymer orpolymerizable species to attach the polymer or polymerizable species tothe internal surfaces of the ceramic oxide network.

A structure also is provided, having a solid-phase three-dimensionalnetwork of ceramic oxide particles that have functional groups bound tosurfaces of the particles. The network of ceramic oxide particles iscross-linked via organic polymer chains that are attached to theparticles via reaction with at least a portion of the surface-boundfunctional groups. The ceramic oxide has the form ZO_(x) where ‘Z’ is ametallic or semimetallic element other than silicon.

BRIEF DESCRIPTION OF THE DRAWINGS

The patent or application file contains at least one drawing executed incolor. Copies of this patent or patent application publication withcolor drawing(s) will be provided by the Office upon request and paymentof the necessary fee.

FIG. 1 is a simplified diagram of the structural formula for silica,illustrated in only two dimensions.

FIG. 2 a is a schematic illustration of the structure of a solid silicanetwork, composed of interconnected strands of secondary particles.

FIG. 2 b is a close-up illustration of the portion of FIG. 2 a indicatedat “2 b.”

FIG. 3 shows a number of epoxy cross-linked silica aerogels made anddescribed in EXAMPLE 1.

FIG. 4 shows SEM micrographs of four epoxy cross-linked aerogels madeand described in EXAMPLE 1.

FIG. 5 is a graph plotting the densities of the thirty-three epoxycross-linked silica aerogel monoliths made and described in EXAMPLE 1.Also shown in the graph is a line indicating the density ofconventional, uncross-linked silica aerogel. Density values (theordinate axis) are given in units of g/cm³.

FIG. 6 is a graph plotting the ultimate strength (stress at break) forthe thirty-three monoliths made and described in EXAMPLE 1. Also shownin the graph is a line indicating the ultimate strength of conventional,uncross-linked silica aerogel.

FIG. 7 is a graph plotting the elastic modulus for the thirty-threemonoliths made and described in EXAMPLE 1. Also shown in the graph is aline indicating the elastic modulus of conventional, uncross-linkedsilica aerogel.

FIG. 8 a is an SEM micrograph of an isocyanate cross-linked silicaaerogel having a bulk density of 450 mg/cm³, made and described inEXAMPLE 2.

FIG. 8 b is an SEM micrograph of an isocyanate cross-linked aerogelhaving a bulk density of 36 mg/cm³, made and described in EXAMPLE 2.

FIG. 9 illustrates two adjacent secondary silica particles linked at aneck region therebetween, and a reaction mechanism for reinforcing theneck region via an isocyanate cross-linking architecture from nativesurface-bound hydroxyl groups on the respective particles.

FIG. 10 shows representative SEM micrographs of several ceramic oxidesthat have been cross-linked using diisocyanate to produce a conformalpolyurethane/polyurea coating over the mesoporous ceramic oxide particlenetwork.

FIGS. 11 a-11 b are SEM micrographs of native and diisocyanatecross-linked VO_(x) aerogels, respectively.

FIG. 12 compares the stress-strain curves for a diisocyanatecross-linked vanadia aerogel and a diisocyanate cross-linked silicaaerogel of similar density as explained in EXAMPLE 5.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

Aerogels are made by extracting liquid from a solvent-filled gel, toleave behind just a solid-phase three-dimensional network of ceramicoxide particles. The wet gel generally is composed of the solid-phaseceramic oxide network of particles just mentioned, whose vast porenetwork is filled and occupied by a liquid phase material. The liquidphase material typically comprises or constitutes the solvent, otherancillary species (water, catalyst, initiator, buffer, etc., if present)and any remaining reactant species for forming the network of ceramicoxide particles via the sol gel process. Essentially, one can think of awet gel and its cognate aerogel as the solvent-saturated solid networkof ceramic oxide particles and the dried ceramic oxide network once thesolvent has been extracted, respectively.

A ceramic oxide is an inorganic compound formed between a metallic or asemimetallic element and oxygen; e.g. silica (SiO₂), titania (TiO₂),alumina (Al₂O₃), etc. Generally, a network of ceramic oxide particlescomprises a three-dimensional structure, with individual nanoparticlesconsisting of atoms of the metallic or semimetallic element(s) linked toone another via interposed oxygen atoms. For example, the networkstructure for silica is illustrated schematically in FIG. 1, though onlyin two dimensions. Because each silicon atom has four free valences,each is linked to four oxygen atoms. Correspondingly, because eachoxygen atom has two donor electron pairs, each of them is linked to twosilicon atoms, except for oxygens on the surface which are linked to onesilicon and one hydroxyl group. The resulting chemical composition ofthe nanoparticles is near to SiO₂. It will be recognized and understoodthat other metallic or semimetallic elements having valences other than+4 (such as silicon) will result in correspondingly different chemicalcompositions in the network of nanoparticles. For example, aluminum hasa valence of +3, resulting in the empirical formula Al₂O₃ for thecorresponding ceramic oxide network. Beyond the foregoing, the onlypractical constraints for producing a ceramic oxide aerogel are that themetallic or semimetallic element must have a valence of at least, andpreferably greater than, +2, and it must be amenable to forming a highlyporous three dimensional network of nanoparticles comprising interposedoxygen bonds, e.g. via a sol gel process through reaction of appropriateceramic oxide precursor species as hereinafter described. Alternatively,other mechanisms to producing such highly porous ceramic oxides can beused.

As evident above, a silica aerogel is prepared by extracting from thepore structure of the solid silica network of nanoparticles the solventin which that network was made (“gelled”). The three-dimensional networkof nanoparticles and the solvent within its pore structure arecollectively referred to as a wet gel, also noted above. Briefly, asilica wet gel is made when an alkoxysilane (typicallytetramethylorthosilicate or ‘TMOS’) is hydrolyzed in an appropriatesolvent, typically methanol, ethanol, acetone, tetrahydrofuran oracetonitrile, to produce the resulting silica gel network and an alkylhydroxide byproduct. The byproduct is methanol when TMOS is used.Alternatively, tetraethylorthosilicate or ‘TEOS’ also can be hydrolyzedto produce a silica gel network, in which case the alkyl hydroxidebyproduct will be ethanol. The silica particles are formed through thelinkage of silicon atoms in the solution (“sol”) via oxygen radicalsformed through hydrolysis and condensation. Thus, the silica gel isformed when the nanoparticles become numerous and coagulate against eachother into a solid three-dimensional network. The gellation to producethe silica network is a form of cross-linking, wherein silica particlesare ‘cross-linked’ via Si—O—Si linkages in neck regions betweenparticles. However, the term ‘cross-link’ and cognates thereof arereserved herein for other polymeric linkages between particles describedhereinbelow.

The hydrolysis reaction of the silicon alkoxide is spontaneous, but itoccurs too slowly for practical applications (gellation can take days orlonger to occur). Hence, it is conventional to employ an acid or a basecatalyst, e.g. an amine catalyst, to accelerate the reaction to a morepractical rate. On hydrolysis and condensation, the resulting solidsilica network is formed having at least two distinct classes or ordersof particles, namely primary particles with densities of 1.7 to 2.2g/cm₃ and secondary particles with densities about half of that of theprimary particles. The primary silica particles are tightly packed,fully dense solid particles having a particle size of less than 2 nm.The secondary particles have a particle size on the order of 5-10 nm,and are microporous, in that they are each made up of an agglomerationof the smaller primary particles. The micropores in the secondaryparticles are provided as the space between the agglomerated primaryparticles that make up each secondary particle. The secondary particlesare arranged to provide an interconnected network of long strands of thesecondary particles to form a mesoporous structure, which strands areoften referred to and known as a “pearl necklace” configuration. Withineach such strand, secondary particles are linked with adjacent particlesvia Si—O—Si bonds across relatively narrow ‘neck regions’ between theparticles. (See FIG. 9, described below). The empty space between thepearl necklace strands of secondary particles is referred to asmesoporocity and accounts for up to 95% of the total volume of the solidnetwork's macrostructure, which is what affords these gels theirdesirable properties.

Once the solid silica network is formed, it is necessary to extract thesolvent from the pore system (meso- and micropores) of the solidnetwork. Historically, this had been difficult to achieve whilemaintaining the structural integrity of the silica gel due to thepresence of the mesopores in the solid network. The liquid-vaporinterface produced on evaporation of liquid within the mesopores wouldexert strong surface tension forces that cause the collapse of the porestructure, causing the solid gels to fracture or shrink, oftenconsiderably compared to their initial size and form. To solve thisproblem, the solvent in the pore system of a silica wet gel is exchangedwith liquid carbon dioxide above its vapor pressure. The resulting solgel, now having liquid CO₂ in the pore system, is heated and pressurizedbeyond the critical temperature and pressure of CO₂, thussupercritically gasifying the CO₂ within the pore system of the solidgel network all at once. The supercritical carbon dioxide is vented,leaving behind the solid silica gel network, thereby producing a driedsilica aerogel whose physical structure is substantially unchanged andundamaged compared to the parent wet gel form. Converting the liquid CO₂directly into supercritical CO₂ prior to venting results in there neverbeing a liquid-gas interface in the mesopores of the gel; hence nosurface tension forces are exerted on the pore surfaces and the solidstructure remains intact.

Other aspects and characteristics of sol gel chemistry, and the processof producing a silica wet gel, and from it a silica aerogel viasupercritical CO₂ venting, are known in the art, and are explained indetail in the aforementioned U.S. patent application publication. Thebrief overview presented here is summary in nature, and will provide abackground from which to understand various embodiments of theinvention, described below.

According to a first embodiment, a functionalized ceramic oxide, such asa functionalized silica, is produced having non-hydroxyl functionalgroups decorated over surfaces of the ceramic oxide network. By‘non-hydroxyl functional group,’ it is meant that the functional groupin question is not simply —OH; the functional group can be a morecomplex moiety that comprises one or more —OH groups. This is not meantto imply the ceramic oxide will have no hydroxyl groups on networksurfaces, as surface hydroxyls are native to ceramic oxides. Rather, aportion of the surface-bound hydroxyls are replaced with other,non-hydroxyl functional groups. These non-hydroxyl functional groupsthen are linked to polymeric or polymerizable species via a suitablereaction mechanism to produce a polymeric chain linking structure. Thepolymeric chain linking structure includes a network of polymer chainsattached to at least one surface-bound non-hydroxyl functional group.The chain linking structure also includes polymer chains attached ateither end to different non-hydroxyl functional groups decorated overthe ceramic oxide surfaces. In addition, individual polymer chains canbe linked to one another via branched chains. In the case of silica,which has a mesoporous macrostructure composed of long interconnectedstrands of secondary particles (which in turn are composed of smallerprimary particles) as noted above, the result is to produce asubstantially conformal coating over the surfaces of the secondaryparticle strands. The conformal coating is composed of the polymer thatis used to link the surface-bound non-hydroxyl functional groups. Theresulting structure, composed of a ceramic oxide backbone coated with aconformal polymeric coating, has only modestly greater density (andlower porosity) than the native ceramic oxide but one to two orders ofmagnitude greater strength. Hence, highly porous ceramic oxide aerogelsnow can be made that are strong enough to withstand anticipatedmechanical loads for numerous practical applications without collapsingor producing dust.

The description that follows is provided primarily with respect to thepreparation of a cross-linked silica aerogel. However, it will beunderstood by persons of ordinary skill in the art that other ceramicoxides can be used based on selection of appropriate ceramic oxideprecursor species that can be reacted to produce a corresponding ceramicoxide network based on another metallic or semimetallic atom, e.g. Al,V, Ti, Zr, etc.

The mechanically strong, organically cross-linked ceramic oxide aerogelswill be best understood through a description of a method by which theycan be made. A functionalized ceramic oxide network is preparedpreferably through a sol gel process similarly as described above. Tointroduce the desired (non-hydroxyl) functionality to the ceramic oxide,a functionalized ceramic oxide precursor that is compatible with sol gelchemistry to produce a solvent-filled gel of ceramic oxide networkparticles via a chemical reaction is copolymerized with anunfunctionalized ceramic oxide precursor via that reaction to producethe particle network. As used herein, an unfunctionalized ceramic oxideprecursor is a species composed of a metallic or semimetallic elementbound to other moieties all through bonds that are labile and subject tobeing broken under the conditions of the particular reaction that is orwill be used to produce the ceramic oxide particle network of the wetgel (sol gel process); i.e. reaction-labile bonds. Conversely, afunctionalized ceramic oxide precursor is a species composed of ametallic or semimetallic element that is bound to at least onenon-hydroxyl functional group via a bond that is not labile (not subjectto being broken) under those reaction conditions (i.e.non-reaction-labile), in addition to at least one, preferably more thanone, other moiety via a bond that is labile under those conditions. Asthe particular chemical reaction proceeds, the solid network ofnanoparticles is formed through copolymerization of both thefunctionalized and unfunctionalized precursor species to produce aceramic oxide wet gel having the desired non-hydroxyl functional groupsattached to the network, in addition to the surface-bound hydroxylgroups that are native to ceramic oxides. As will be seen, at least aportion (probably a significant proportion) of the non-hydroxylfunctional groups are surface-bound on the secondary ceramic oxideparticles, probably displacing (taking the place of) a proportionatenumber or quantity of hydroxyl groups.

Next, these functional groups are linked to other polymerizable orpolymeric organic species to “cross-link” the ceramic oxide network viaorganic polymer chains. Finally, the wet gel comprising the cross-linkedceramic oxide network is dried via an appropriate process to produce thefinal cross-linked ceramic oxide aerogel. This process, described herein summary, will now be more fully described below with respect tosilica.

To prepare a cross-linked silica aerogel, first the corresponding silicawet gel is prepared by hydrolyzing an alkoxysilane such as TMOS or TEOSto produce the wet gel having a solid silica particle network similarlyas described above. However, in addition to the alkoxysilane, which isan unfunctionalized silica precursor species, a functionalized silicaprecursor species also is included in the hydrolysis reaction. Like thealkoxysilane species, the functionalized silica precursor speciesincludes a silicon atom bound to at least one, preferably to at leasttwo, most preferably to three, other moieties via a hydrolysable bond(i.e. a bond that is labile under the particular reaction conditions),so the silicon atom can be integrated into the silica network during thehydrolysis reaction, e.g. with TMOS or TEOS. As used herein, ahydrolysable bond is one that is labile and subject to being brokenunder hydrolysis conditions employed to produce the solid silica networkin the presence of water as a reactant, and a suitable catalyst ifappropriate, so that the atoms linked by the hydrolysable bond becomedissociated from one another. The moieties linked to the silicon (orother metallic or nonmetallic) atom via hydrolysable (labile) bonds arereferred to as leaving groups, because following hydrolysis (or whateverthe particular sol gel reaction used) they will be dissociated from(they will ‘leave’) the silicon or other metallic or semimetallic atom,and consequently will not be part of the resulting network.

In addition to the leaving group(s) attached to the silicon atom, thefunctionalized silica precursor species also has at least onenon-hydroxyl functional group attached to the Si atom via anon-hydrolysable (i.e. non-labile) bond. A non-hydrolysable bond is onethat is not subject to being broken under the hydrolysis conditionsnoted above.

The alkoxysilane and functionalized silica precursor species arecombined and reacted with water under appropriate hydrolysis conditionsdescribed above to copolymerize them and produce a gelled network ofsilica particles comprising silicon atoms from both of the precursorspecies. The resulting network is highly porous (volume void fraction ofat least 80, preferably 85, more preferably 90, most preferably 95,percent, or higher). It includes the conventional silicon-oxygenlinkages for silica gels, but in addition has a plurality ofnon-hydroxyl functional groups bonded to the silicon atoms that weresupplied from the functionalized precursor species.

In one embodiment, the functionalized silica precursor species is3-aminopropyltriethoxysilane or ‘APTES.’ Like TMOS, APTES has threealkoxy moieties (ethoxy groups) linked to the central silicon atom via ahydrolysable bond. However, APTES also includes a fourth moiety, a3-aminopropyl group, that is linked to the silicon atom via anon-hydrolysable Si—C bond. Under hydrolysis conditions, the threealkoxy bonds in APTES are broken and the associated ethoxy groupsconverted to ethanol. This frees three bonding sites on the silicon atomthat now can be linked to oxygen atoms in the silica network, while thelatter, fourth bond is not broken during hydrolysis. Consequently, theAPTES-source silicon atom will continue to carry the 3-aminopropylmoiety, with the terminal —NH₂ functional group, after it is integratedin the silica network. In the case of silica formed from copolymerizingan alkoxysilane and APTES, it has been found the resulting solid networkexhibits the same basic hierarchical structure described above, havingmicroporous secondary particles (particle size ˜5-10 nm), composed ofagglomerations of smaller and highly dense primary particles (particlesize ˜<2 nm), linked in long interconnected strands to produce aninterconnected pearl necklace structure. It has been found that a largeportion of the aminopropyl-linked silicons are located at the surfacesof the secondary particles, with the aminopropyl groups decorated overthe secondary particle surfaces and extending into the superjacent voidspace (mesopores). It is noted that in the case of APTES, it isunnecessary to incorporate a separate catalyst into the hydrolysisreaction because the amino groups on APTES provide more than adequatebasic character to the sol to catalyze the hydrolysis reaction. In fact,it has been necessary in experiments (described below) to cool theTMOS/APTES solution/water mixture to slow the gellation rate and permitpouring of the sol into a desired mold prior to substantial gellation.

It is believed, and experimental results (also described below) havesuggested, that integration of the APTES-source silicon atoms at thesecondary particle surfaces is favored compared to intra-particleintegration within the silica network. There are several potentialexplanations for the apparent preference of the aminopropyl-linkedsilicon atoms to be incorporated into the ceramic oxide network atsecondary particle surfaces. First, hydrolysis of the alkoxy groups ofAPTES is slower than that of TMOS. Also, the primary particles are fullydense, having substantially no porosity. The relatively bulkyaminopropyl group (NH₂—CH₂—CH₂—CH₂—) would be strongly stericallydisfavored compared to the much more compact oxygen linkage (—O—) withinthe fully dense primary particles. Thus, the APTES-source silicon atoms,having the non-hydrolysable aminopropyl group and therefore one lessbonding site compared to the fully hydrolyzed silicon atoms fromalkoxysilane, may tend to terminate network growth or linkage. If thesesilicon atoms were concentrated internally, they might be expected todisrupt gellation and the formation of a uniformly dense and fullyexpansive solid silica network. Experimental evidence suggests thatAPTES itself does not gel. Hence both steric considerations and the lackof a fourth bonding site compared to the alkoxysilane-source siliconatoms suggest the APTES-source silicons would be relatively disfavoredinternally, within either the primary or the secondary silica particles.

However, such constraints are not present at the surfaces of thesecondary particles. The secondary particle surfaces define a vastnetwork of relatively large mesopores that easily can accommodate thesteric bulk of aminopropyl groups concentrated at and extending fromthose surfaces. Furthermore, because the secondary particles are linkedto one another only in relatively narrow neck regions to form theabove-mentioned pearl necklace structure, silica network propagationabove the secondary particle surfaces for the most part does not occur,and there is less need for a fourth Si-bonding site. For all thesereasons the incorporation of the APTES-source silicon atoms at thesurfaces of secondary particles within the silica network may bethermodynamically favored compared to intra-particle integration ofthese silicons.

Following hydrolysis of TMOS and APTES as described above, the resultingwet gel framework comprises a solid silica network having a structureillustrated schematically in FIGS. 2 a and 2 b. (Neither FIG. 2 a norFIG. 2 b is drawn to scale, and both are for illustrative purposesonly). FIG. 2 a is a schematic illustration of the structure of a solidsilica network, composed of interconnected strands of secondaryparticles defining a large mesoporous network. Adjacent secondaryparticles in a chain are connected to one another (via —Si—O—Si—linkages between the surfaces of adjacent particles) at relativelynarrow neck regions as illustrated in the figure. It is believed thatthe brittleness and low structural strength of native silica aerogels isthe result of these neck regions, which may be prone to fracturerelatively easily. FIG. 2 b is a close-up illustration of several of thesecondary particles in FIG. 2 a, showing schematically the aminopropylfunctional groups extending from the surfaces of the secondaryparticles. These functional groups remain attached to their respectiveAPTES-source silicon atoms, which now have been incorporated into thesilica network at the surfaces of the secondary particles. The number,concentration and arrangement of illustrated aminopropyl groups is notto be taken or interpreted literally, or to convey any information aboutthe actual surface concentration of these groups. The purpose of FIG. 2b is simply to illustrate that the secondary particles are providedhaving aminopropyl groups decorated over their surfaces as a result ofthe copolymerization of TMOS and APTES via hydrolysis as describedabove. Of course, terminal hydroxyl groups also will be present on thesurfaces of the secondary particles, however these are not illustrated.

Once the wet gel network having the aminopropyl groups decorated overthe secondary particle surfaces has been prepared, the terminal aminescan be further reacted with a polymer or a polymerizable species, orother species that can serve as a base for linking or forming a polymerchain to the secondary particle surfaces as part of a polymercross-linking structure between secondary particle strands in the solidsilica network. It is noted that while the solid silica network itselfmay be considered a polymer produced from the copolymerization of TMOSand APTES, the term ‘polymer’ is reserved herein to refer to different,preferably organic, polymeric species or chains, non-native to a ceramicoxide network, that link or which are provided to cross-link thatceramic oxide network. The term ‘cross-link’ and cognate terms such as‘cross-linked,’ cross-linking' and the like herein refer to linkagescomposed of polymeric structures non-native to the ceramic oxide networkthat extend between or link, or which are provided to link, differentportions or points within that ceramic oxide network.

Returning to the present embodiment, the functionalized ceramic oxidenetwork resulting from the co-hydrolysis of TMOS and APTES is an amino-(or aminopropyl-) functionalized solid silica network, having aminogroups decorated over the surfaces of the secondary particles within themesopores. At this point, the silica network is in the form of a sol gelwhose porous structure is filled with the hydrolysis solvent, hydrolysisreaction byproducts (such as MeOH from TMOS, EtOH from the ethoxy groupsof APTES as well as from TEOS if used, etc.) and other unconsumedspecies.

To produce a polymer cross-linked wet gel, the functional (amino) groupson the surfaces of the secondary particles are reacted with anappropriate monomer or other species for forming or linking to acompatible polymer chain. For example, a diisocyanate can be linked tothe terminal amino groups on the surfaces of the secondary particles viaa urea linkage according to equation (1).

The resulting terminal isocyanate group, now attached to the secondaryparticle surface via the urea linkage, can be reacted (polymerized) withadditional polyisocyanate groups to produce a polyurea polymerstructure, e.g. as in Eq. 2.

To drive this polymerization reaction, water adsorbed on the silicasurfaces may be sufficient, otherwise water can be added to the sol. Theterminal isocyanate group in the product of Eq. 2 above likewise can bereacted with an amino group at the surface of the same or a differentsecondary particle to produce a polyurea linkage or ‘cross-link’ betweentwo different secondary particles, for example between adjacentsecondary particles in the neck region between them, or betweendifferent sites on the same secondary particle, Eq. 3.

Alternatively, the above polymerization (cross-linking) reactions can becarried out with an isocyanate that is greater than 2-functional (i.e.having more than two functional NCO groups. For example, 3- and4-functional isocyanates also can be used. It will be understood thatjust as in conventional polyurethane chemistry where isocyanates areprevalent, the greater the isocyanate functionality the more highlybranched the resulting cross-linking polymeric structure will be, andconsequently the more rigid and inflexible the resulting cross-linkedceramic oxide (silica) network. However, in applications whereflexibility is of little concern, highly branched cross-linkingstructures may be desired to impart greater strength to the ultimatesilica aerogel product (produced after the cross-linked sol gel isdried). This added strength will come at a cost in terms of a smallincrease in weight, however, because a more highly cross-linked aerogelwill be more dense compared to uncross-linked aerogel.

It is noted that in the above mechanism where isocyanates are used toprovide the cross-linking polymer chains, hydroxyl groups present on thesurfaces of the secondary particles (terminal silanols) also may reactwith —NCO groups to produce a carbamate (urethane) linkage:

which is more akin to the conventional polyurethane reaction mechanismbetween polyisocyanates and polyols. However, the linkages formedbetween polyisocyanate molecules within the polymer chains, more distalfrom the surface-bound silanol groups, will still be urea linkages asabove. Thus, in the present embodiment, polyurea chains may be linked ateither end to respective silanol groups (carbamate linkage), aminogroups (urea linkage) or one of each. Still further, polyurea chains maybe linked only at one end to either a silanol or an amino group, withthe opposite end of the polymer chain being unlinked to a secondaryparticle surface. In all of the above alternatives, polyurea chains maybe linked together via branched polyurea chains, with the degree ofbranched linkages between the polymer chains depending in part onwhether a 2-, 3- or 4-functional isocyanate is used for polymerization.Alternatively, mixtures of polyfunctional (2-, 3- and/or 4-functional)isocyanates also can be used. It will be further recognized there is thepotential for additional cross-linking involving further reaction ofsecondary amine (—NH—) groups in carbamates or polyurea to form tertiaryamines (—N<) (allephanates and biurets, respectively), as is common toall polyurethanes.

In the foregoing reactions, ‘R’ can be any group or moiety to which oneor multiple —N═C═O groups can be attached, as the individual case maybe. For example, ‘R’ can be or include a straight or branched alkyl oraryl group, aromatic group, olefinic group, or any combination of this,with or without additional functional species, so long as suchadditional functional species will not intolerably interfere with theformation of urea linkages between isocyanate groups on differentmonomers, or between an isocyanate group and a surface-bound amine groupin the ceramic oxide network. It is contemplated that ‘R’ can beprovided or designed to impart additional desirable characteristics tothe resulting polymer cross-linked ceramic oxide aerogel, for exampleincorporating additive functional groups as described more fully below.

As a further example, a polyepoxide also can be linked to a terminalamino groups on the surfaces of a secondary particles via an epoxylinkage as shown in Eq. (4).

In Eq. 4, a trifunctional epoxide (N,N′-diglycidyl-4-glycidyloxyaniline)is reacted with the terminal amino group at the surface of the secondaryparticle. This results in a difunctional epoxide moiety attached to asecondary particle surface. Each of the epoxide groups of thisdifunctional epoxide moiety in the product of Eq. 4 can react(polymerize) with a) a yet-unreacted terminal amino group at the surfaceof the same or a different secondary particle, the latter resulting ininter-particle cross-linking, or b), at temperatures above 150° C.,other epoxide groups attached to the surface of the same or a differentsecondary particle. An exemplary mechanism involving the difunctionalepoxide product in Eq. 4 bound to each of two secondary particles, and athird secondary particle having an as-yet unreacted surface-bound aminogroup, is illustrated in Eq. 5.

It will be understood that the exemplary mechanism shown in FIG. 5 ismerely illustrative of numerous combinations of epoxide-epoxide andepoxide-amine reactions that are possible to produce a three dimensionalpolymeric epoxy network structure. In addition, it will be understoodthat di-, tetra-, or other polyfunctional epoxides also can be used, orcombinations of them with each other or with tri-functional epoxidessuch as the one described above. For example, the following polymernetwork architecture using 1,3-diglycidyloxybenzene as a difunctionalepoxide monomer, Eq. 6, also is within the scope of the invention:

A cross-linked epoxy polymeric network is produced via epoxy linkagesbetween epoxide groups on different polyepoxide monomers (attemperatures above 150° C.), as well as between such groups andsurface-bound terminal amino groups within the ceramic oxide particlenetwork. (Epoxies also will react with SiOH surface groups, although toa much lesser extent.) The result is an epoxy cross-linked solid ceramicoxide (silica) network, in the form of a wet gel whose pore structure issaturated with the solvent used to carry out the epoxy polymerizationreactions. Analogous with the polyisocyanate network discussed above, anepoxy polymeric network will provide epoxy linkages or ‘cross-links’between different secondary particles, for example between adjacentsecondary particles in the neck region between them, or betweendifferent sites on the same secondary particle. At elevated temperaturesor in the presence of catalyst, branched epoxy linkages between epoxypolymer chains are also possible. Under conditions that have beenemployed to produce the epoxy cross-linked silica networks describedhere (no catalyst and relatively low temperatures), epoxides do nottypically form large networks or chains of epoxy oligomer (monomer).Hence, under these conditions the resulting epoxy cross-linked silicanetwork is primarily an epoxy monolayer over the surface of thesecondary particle strands (pearl necklaces).

The foregoing discussion has been provided with respect to severalspecific di- and tri-functional polyepoxides. However, it will beunderstood that other polyfunctional epoxides having the general form:

also could be used, where ‘R’ is or can be or include any structurecompatible with the epoxy cross-linking chemistry, similarly asdescribed above.

In still a further example of another polymeric cross-linkingarchitecture, a styrene-containing species also can be linked to theterminal amino groups on the surfaces of the secondary particles via anappropriate linkage. The attached styrene group then can be reacted(polymerized) with other styrene-containing monomers to produce apolystyrene cross-linked polymeric network. For example, Eq. (7) belowillustrates a reaction for attaching a styrene group to a surface-boundterminal amino group attached to a secondary particle of the ceramicoxide network.

In Eq. 7, the styrene-containing species used to attach the styrenegroup to the terminal amine is 4-vinylbenzyl chloride, which contains astyrene moiety as shown in Eq. 7. This species is convenient because theterminal chloride reacts readily with the amine to link the amine andthe residual p-methylstyrene moiety, producing HCl as a byproduct. Inaddition, other suitable styrene-containing species, having otherfunctional groups that will react with the amino group to attach thestyrene group to the ceramic oxide, can be used, (styrene functionalizedepoxides, etc.). However, the preferred method is to co-polymerizep-trimethoxysilyl-styrene with TMOS, analogously to the copolymerizationof APTES (though amine catalyst is necessary) to apply the styrenemoiety directly to the surface of the nanoparticles.

Once styrene groups have been bound to internal surfaces of the solidceramic oxide network, they can be reacted (polymerized) with otherstyrene monomers to produce a polystyrene cross-linked polymer network,for example as illustrated in Eq. 8. The styrene functionalized gels areplaced into solutions containing the monomers of choice and AIBN as theinitiator, and the polymerization occurs at elevated temperatures suchas 75° C., or 75° C. to 100° C.

It will be understood that Eq. 8 is merely illustrative of a possiblepolystyrene cross-linking mechanism using native, non-functionalizedstyrene monomer to provide the polymeric cross-links. The actualcross-linked polystyrene network produced through polymerization ofstyrene monomers will include polystyrene chains of varying lengthdepending on the concentration of additional monomer, extending betweenamino groups at the surfaces of different secondary particles, as wellas between such groups attached to the surface of the same secondaryparticle. As with the previously described cross-linking species,polystyrene chains also may be provided between two adjacent secondaryparticles linked at a neck region therebetween in the same strand, aswell as between different secondary particles in the same or indifferent strands. Also, alternatively to non-functionalized styrenemonomer, other functionalized styrene-containing monomeric species (e.g.of the form R-[styrene], or R-[styrene]_(n) also could be used.Similarly as before, ‘R’ can be or include any structure compatible withstyrene polymerization to produce a polystyrene cross-linkingarchitecture. Examples of styrene-containing monomeric species that havebeen successfully used to produce polystyrene cross-linked silicaaerogels are 4-vinylbenzyl chloride and pentafluorostyrene, as well asmixtures thereof (see EXAMPLE 3). It will be understood that to producea branched polystyrene network, it may be necessary or desirable toincorporate at least some functionalized styrene-containing monomers, orotherwise monomers containing at least two styrene groups. Otherwise,pure, non-functionalized styrene may produce primarily straight andunbranched polystyrene chains as known in the art.

Also, generally it is desirable to utilize a radical initiator speciesto induce styrene polymerization. In the mechanism illustrated in Eq. 8,azobisisobutyronitrile (AIBN) is employed as a radical initiator.However, other suitable radical initiators can be employed, e.g.peroxy-based initiators including benzoyl peroxide can also be utilizedunder similar thermal conditions to obtain the polymerization.

As a further example, a polyamic acid also can be linked to terminalamino groups on the surfaces of the secondary particles via an anhydridelinkage as shown in Eq. (9).

In Eq. 9, a polyamic acid terminated with anhydride is reacted withterminal amino groups at the surface of the secondary particles. Thisresults in amic acid moieties attached to secondary particle surfaces.Each of the anhydride groups shown in Eq. 9 can be reacted (polymerized)with a yet-unreacted terminal amino group at the surface of the same ora different secondary particle. Subsequent heating at temperatures of150-200° C. promotes imidization giving a thermo-oxidatively stablecross-link. Incorporation of a trifunctional amine in the polyamic acidas shown in Eq. 10 allows n attachments of surface amines to anhydridesalong oligomer backbones.

In addition to cross-linking the ceramic oxide via the isocyanate-,epoxy- and styrene-based polymer chains described above, other polymerarchitectures also could be used for cross-linking. The first step willbe to select an appropriate base species or monomer to be attached tothe ceramic oxide to support the desired polymer architecture. Forexample, a moiety including a free carboxylic acid group can be attachedto the ceramic oxide if it is desired to produce a polyestercross-linking architecture. Alternatively, other functional groups alsocould be decorated to the ceramic oxide network, such as other olefins(polyolefin cross-linking architecture), alkyl or aryl halides(polybenzoxazole cross-linking), ketones or aldehydes (polybenzimidazolecross-linking), ethers (polyether cross-linking), etc. It should beevident that once the appropriate base or monomeric species has beendecorated to the ceramic oxide network's internal surfaces,cross-linking is a matter of performing essentially the conventionalcross-linking reactions, using conventional reaction conditions,associated species (such as catalysts and initiators), solvents, etc.,with care taken to select conditions, solvents, etc. compatible with thesolid ceramic oxide network. However, except for terminal hydroxylgroups, ceramic oxides are highly stable, inert materials, so there willbe few instances where selection of appropriate cross-linking speciesand/or conditions will be impacted by the particular ceramic oxide solgel to be polymer cross-linked.

It also will be evident that attachment of the desired base or monomericspecies to the ceramic oxide network, e.g. to decorate the particlesurfaces of such network with the functional groups that will supportthe desired cross-linking chemistry, will be a matter of routine basedon the present disclosure. Broadly, one selects or designs a moleculehaving the desired reactive species to support cross-linking via thedesired architecture, but also having another reactive site that can beused to attach the molecule (having the reactive species) to the ceramicoxide network. In the case of amine-decorated silica as above, thementioned functional group is an amine, and the other reactive site isan (or more than one) alkoxysilane(s), e.g. three ethoxysilyl groups onAPTES. Alternatively, if the native hydroxyl groups on ceramic oxidesurfaces are to be used, then the reactive site should be reactive withsuch terminal hydroxyl groups to produce an analogous result. It will beunderstood that the cross-linking supportive functional group and theother reactive site to attach the molecule to the ceramic oxide networkcan be the same (as for the polyfunctional epoxides described above), orthey can be different (as for 4-vinylbenzyl chloride, also describedabove).

Still further, the above cross-linking methods are not to be limited tosilica (SiO₂), as the methods and reactions described herein to producea cross-linked ceramic oxide are also applicable to other ceramicoxides. For example, the methods and reactions described herein alsocould be used to produced other polymer cross-linked ceramic oxides,including but not limited to titania, vanadia, manganesia, zirconia,ruthenia, alumina, iron oxide, india (indium oxide), yttria, europia,etc., (which can be represented as TiO_(x), VO_(x), MnO_(x), ZrO_(x),RuO_(x), AlO_(x), FeO_(x), InO_(x), YO_(x), EuO_(x), etc.,respectively). All of these ceramic oxides will have terminal hydroxylgroups present on internal surfaces thereof, which form the respectiveceramic oxides and define their mesoporous networks. Accordingly, all ofthese, as well as other ceramic oxides, can be cross-linked as describedherein by using the surface-bound hydroxyl groups to attach the desiredmoiety to support the chosen cross-linking architecture. Alternatively,other ceramic oxides besides silica also can be produced havingnon-hydroxyl functional groups decorated to the internal surfacesthereof. Specifically, such other ceramic oxides can be produced viaappropriate reactions analogous to the TMOS-APTES hydrolysis reactionfor producing amine-decorated silica, with suitable ceramic oxideprecursors based on the associated metallic or semimetallic element,“Z,” having leaving groups attached to a central Z atom via reactive orreaction-labile bonds, and functional groups attached via non-reactiveor non-reaction-labile bonds. For example, an alkoxy-Z and afunctionalized ZO_(x) precursor species, (a generic ceramic oxide beingrepresented as ZO_(x)), having at least one functional group attached tothe Z-atom via a non-reactive bond can be prepared by persons havingordinary skill in the art for the desired metallic or nonmetallic “Z”atom, and copolymerized via a sol gel process employing an appropriatereaction for producing a gelled ceramic oxide network decorated with thedesired functional group. Alternatively, the ceramic oxide mesoporousnetwork can be decorated with appropriate functional groups bypost-gellation treatment with molecules having the desired functionalgroups that will support cross-linking via the desired chemistry, butalso having another reactive site that can be used to attach themolecule (having the reactive species) to the ceramic oxide network. Inthat regard, such reactive site that can be used to attach said moleculeto the ceramic oxide network can be based on the same metallic orsemimetallic element “Z” as the ceramic oxide mesoporous network, oranother element. For example, the mesoporous surfaces of a metallic orsemimetallic ceramic oxide mesoporous network can be decorated withamines by post-gellation treatment with APTES, or it can be modifiedwith styrene by post-gellation treatment with p-trimethoxysilyl styrene.It is expected that not all metal and semimetal oxide mesoporousnetworks will be amenable to all these methods, and it is recognized andexpected that some experimentation may be necessary to determine whetherfunctionalized wet gel networks based on other, non-Si metallic orsemimetallic elements can be prepared via sol gel chemistry to produce agelled solid network. In addition, some ceramic oxides may require acidor base catalysis, different reaction conditions such as hightemperature for endothermic reactions, low temperature for highlyexothermic reactions, appropriate solvents, etc. However, suchexperimentation is well within the ability of a person having ordinaryskill in the art, and will be routine based on the present disclosureonce the desired ceramic oxide and the functional groups to be decoratedthereto have been identified for producing an aerogel with the desiredproperties.

Regardless of what polymeric architecture or what ceramic oxide isemployed to produce a polymer cross-linked solid ceramic oxide network,the next step is to remove the liquid (solvent) from that network toproduce a dried, cross-linked aerogel from the corresponding wet gel.Conventionally, aerogels are prepared from the corresponding wet gel viaa CO₂ exchange and supercritical drying procedure as describedpreviously. That same procedure can be used for the present cross-linkedwet gels to produce the cross-linked aerogel. Alternatively, at least insome cases supercritical drying is not necessary for polymercross-linked aerogels, as it had been for conventional aerogels, toprevent shrinkage or destruction of the ceramic oxide network onevaporation of the solvent from its pore structure. This is because thecross-linked polymeric network imparts adequate structural strength andsupport to reinforce the ceramic oxide network and prevent itsdestruction from surface tension forces generated as the liquid withinthe pores evaporates. For example, as described in publication No.2004/0132846, an isocyanate-linked silica sol gel (from nativesurface-bound hydroxyl groups) has been dried to produce thecorresponding aerogel via exchange of the cross-linking solvent withpentane, and then heating to elevated temperature to evaporate theliquid pentane from the pore structure. The same procedure also shouldapply to the aerogels of the presently described embodiment, wherein anon-hydroxyl functional group is decorated to the ceramic oxide particlesurfaces and then used as a base to launch the cross-linking polymernetwork.

In addition to amine functional groups, other functional groups also canbe decorated to the surfaces of ceramic oxide particles to provide abase for polymer cross-linking of the ceramic oxide network, e.g. byincorporation into the functionalized ceramic oxide precursor speciesmentioned above via a non-reactive or non-reaction-labile bond.

In addition to the polymerizable moieties on the monomeric species usedto cross-link the surface-bound functional groups either present orincorporated into a ceramic oxide network, such species also can haveother chemical moieties effective to impart a desired property to thefinished polymer cross-linked ceramic oxide aerogel composition. Forexample, any of the aforementioned monomeric species (epoxides, styrenesand isocyanates), as well as other monomeric species capable to supportother cross-linking architectures, can have, e.g., attached phosphategroup(s), which are known to impart flame retardant qualities tomaterials. As a further example, conducting polymers having free valenceelectrons capable to act as charge carriers can be incorporated into themonomeric species to impart a degree of electrical conductivity to thefinished aerogel. As a still further example, catalytic species also canbe incorporated in this manner. Other groups or moieties capable toimpart other desired physical, chemical, electrical, magnetic,non-linear optical or other properties also can be incorporated in thismanner, which incorporation would be within the ability of a personhaving ordinary skill in the art. Such groups that can be soincorporated into the polymer cross-linked aerogels throughincorporation in monomeric (polymerizable) species used forcross-linking are broadly referred to herein as ‘additive functionalgroups’ because they impart added characteristics to the final aerogel.Alternatively, such groups can be added either by post-gellationtreatment, or by post-cross-linking treatment with appropriateattachable molecules.

As will become apparent in the following examples, a mixture ofdifferent monomeric species having different overall structures also canbe used to produce an aerogel so long as the different monomers havecommon or compatible polymerizable moieties; e.g. a styrene moiety or anepoxy moiety, etc. For example, the degree of flexibility of a polymercross-linked ceramic oxide aerogel will depend, among other things, onthe prevalence of rotatable bonds in the cross-linked polymer structure.Much rotation is possible for carbon-carbon or carbon-heteroatom singlebonds in a polymer chain. Alternatively, higher order (double or triple)carbon-carbon bonds, a high degree of cross-linking as well as aromaticrings in the polymer chain do not permit significant rotation.Consequently, a polymer network composed primarily of highlycross-linked isocyanate (polyurea), or based on highly unsaturatedmonomers (significant degree of higher order bonds), will produce arelatively rigid, inflexible aerogel. If it is nonetheless desired toproduce a more flexible cross-linked aerogel based on isocyanatecross-linking chemistry, then a mixture of polyisocyanate monomers, e.g.one monomer having few or no rotatable bonds and another having longerchains between cross-links, can be used, and their ratio tuned toachieve a desired degree of flexibility. Of course, the total availablerange of available flexibility may be constrained within broad limitsbased on the cross-linking architecture selected, but some degree ofadjustment should be possible through this technique.

Conversely, a cross-linked network based on epoxy chains, havingnumerous ether linkages, which permit a high degree of rotation, will besubstantially more flexible, and its flexibility may depend on theproportion of ether linkages in the network. Still further, etherlinkages can be provided in other, non-epoxy monomers, such aspolyisocyanate monomers or polyimides using amine terminatedpolyalkyleneoxides in the polymer chain as shown in Eq. 9 above, toimpart a greater degree of flexibility to the resulting aerogels asdescribed above. At this point, it should be evident that a personhaving ordinary skill in the art will be able to design a wide varietyof cross-linking architectures, based on a variety of polymerizablespecies, additive functional groups, other structural linkages andmoieties within the polymerizable species, etc., to produce aerogelshaving characteristics suitable to any number of potentially desirableapplications. There is virtually no limit to the applications for thepotential variety of polymer cross-linked ceramic oxide aerogels thatcould be prepared by a skilled person based on the present disclosure.

Further aspects of the present invention will be illustrated andunderstood in the context of the following examples, which are providedby way of illustration and not limitation.

Example 1 Epoxy Cross-Linked Silica Aerogels from Amine-Decorated SilicaNetwork

Experiments were conducted to determine the effects of varying the APTESto TMOS ratio for producing an amine-decorated solid silica network thatwas subsequently cross-linked via an epoxy architecture. Specifically,the APTES to TMOS ratio was varied, and the resulting sol gels weretreated with di-, tri-, or tetra-functional epoxides in variousconcentrations. The reaction conditions (time and temperature) werevaried along with the APTES concentration, and the epoxy type andconcentration. The resulting cross-linked gels were dried withsupercritical carbon dioxide to produce the corresponding aerogels,which were characterized macroscopically, microscopically andchemically. The most distinct property of the polymer cross-linkedaerogels was their greatly improved mechanical properties compared tonative aerogels. In the present experiments, the strengths of theseaerogels were measured as a function of the chemical composition,processing conditions, and resulting density.

Experimental Methods

As noted above, in these experiments di-, tri- and tetra-functionalepoxides were used as the cross-linking moieties to produce the epoxycross-linking architecture. The particular species of each of theseepoxides were as follows:

Difunctional epoxide: diglycidylaniline,

Trifunctional epoxide: N,N′-diglycidyl-4-glycidyloxyaniline, and

Tetrafunctional epoxide: 4,4′-methylenebis(N,N′-diglycidylaniline).

Solvents used in the production of cross-linked epoxy aerogels includedmethanol (ACS reagent grade), tetrahydrofuran (THF:ACS reagent grade),anhydrous tetrahydrofuran (ACS reagent grade purified and dried using asolvent purification system by Innovative Technology, Inc.), and acetone(ACS reagent grade). Supercritical drying was performed using liquidcarbon dioxide (standard T type tank with a siphon tube, AGA Inc.,Cleveland Ohio).

All of the wet sol gels were prepared by mixing together two solutions,A and B. Solution A contained a variable volume ratio of APTES to TMOSin methanol solvent; solution B contained an equal amount of methanol,the appropriate amount of water, and a gellation catalyst, if necessary.For comparison, wet sol gels without APTES were prepared by using TMOS,deionized water, and methanol in the molar ratio of 1:3:8.5; gellationwas catalyzed

by adding 6 μmol of NH₄OH (30%) for every mol of TMOS. Thus, in atypical procedure solution A contained 4.5 mL of methanol and 3.85 mL ofTMOS. Solution B contained 4.5 mL of methanol, 1.5 mL of water, and 40μL of the NH₄OH solution. Wet gels with APTES were prepared by replacing25% or 50% v/v of TMOS in solution A with the corresponding volume ofAPTES. Importantly, in these reactions the base catalyst, NH₄OH, wasable to be eliminated from the sol gel hydrolysis reaction to producethe solid silica network because the amine group of APTES itself servedas the gellation catalyst. In fact, it was necessary to slow down theAPTES-catalyzed gellation reaction long enough to allow pouring the solinto the appropriate molds. Thus, solutions A and B were cooled in a dryice-acetone bath prior to mixing when APTES was included in thehydrolysis reaction. Immediately after mixing the cold solutions, 3.6 mLof each mixture was poured into cylindrical molds approximately 1 cm indiameter and 4 cm in length to form the desired cylindrical monolithsfor testing. The molds were covered with Parafilm and the gels wereallowed to age for 48 hours to fully form the respective solid silicanetworks.

Subsequently, the wet sol gels were carefully removed from the molds andwashed approximately every 12 hours with methanol (20 mL) a total offour times to remove NH₄OH if present, but was done for the sake ofconsistency for all gels. The sol gels were then washed four more timesevery 12 hours, twice with THF (20 mL) and twice with anhydrous THF (20mL).

Three different volume percent concentrations for each epoxide inanhydrous THF (15%, 45%, and 75% v/v) were used for cross-linking.Because of the high viscosity of the epoxides, dissolution wasfacilitated with heating at ca. 50° C. and vigorous shaking. Once thesolutions were homogeneous, the sol gels were placed into the desiredepoxide solution (20 mL), gently stirred, and allowed to equilibrate for48 h. Subsequently, and while still in the corresponding epoxidesolution, wet gels were placed in an oven and allowed to react atspecified temperatures (50, 72.5, or 95° C.) and lengths of time (16,44, or 72 h) as predetermined by a statistical experimental designprocedure. Afterward, cross-linked sol gels were allowed to cool to roomtemperature and each gel was first washed four times with anhydrous THF(20 mL each time) in 12 hours intervals to remove the unreacted epoxide,and then four times with acetone (20 mL each time). After the finalacetone wash, monoliths were transferred together with the final acetonewash solution into an autoclave and were dried from supercritical CO₂(40° C. and ˜100 bar) to produce the respective dried aerogels.

Chemical characterization of the epoxy cross-linked aerogel monolithswas conducted by infrared, ¹³C NMR, and ²⁹Si NMR spectroscopy. The dryaerogel samples were ground by ball-milling for 5 min using a SPEX 5300Mixer Mill. Solid samples were mixed with KBr and pressed into a pellet,and infrared spectra were obtained with a Nicolet-FTIR SpectrometerModel 750. Solid ¹³C NMR and ²⁹Si NMR spectra were obtained on a BrukerAvance 300 Spectrometer, using cross-polarization and magic-anglespinning at 7 kHz. The acquisition also employed spinning-sidebandsuppression using a CPSELTICS sequence. The solid ¹³C spectra wereexternally referenced to the carbonyl of glycine (176.1 ppm relative totetramethylsilane, TMS). The solid ²⁹Si spectra were externallyreferenced to the silicon peak of the sodium salt of3-trimethylsilylpropionic acid (0 ppm).

Physical characterization of native and cross-linked silica aerogelsamples was conducted by thermogravimetric analysis (TGA), scanningelectron microscopy (SEM), and nitrogen-adsorption porosimetry. TGA wasperformed using a TA Instruments Model 2950 HiRes instrument. Sampleswere run at a temperature ramp rate of 10° C. min-1 under nitrogen orair. Samples for SEM were coated with gold and microscopy was conductedwith an Hitachi S-4700 field-emission microscope. Fornitrogen-adsorption porosimetry, samples were outgassed at 80° C. for 24h under vacuum and studies were conducted with an ASAP 2000 SurfaceArea/Pore Distribution analyzer. Three-point flexural bending tests wereperformed according to ASTM D790, Procedure A (Flexural Properties ofUnreinforced and Reinforced Plastics and Electrical InsulatingMaterials), using an Instron 4469 universal testing machine frame with a2 kN load cell and a three-point bend fixture, with 0.9 in. span and 25mm roller diameter. Typical samples were cylindrical, ˜1 cm in diameterand ˜4 cm in length. The crosshead speed was set at 0.04 in. min-1.Three samples were tested for each composition and the average of thethree values are presented in Table 1 below.

Dielectric characterization also was conducted by measuring thecapacitance of thin (˜9 mm diameter, ˜1 mm thick) disks. The thin diskswere cut from cylindrical monoliths with a diamond wafer blade and saw.The lateral area of each disk was masked with duct tape and the top andbottom surfaces were sputter-coated with gold using a Baltek MED 20Sputtering System. Copper wires were Scotch-taped on the two surfacesand were connected to an MFJ-259B SWR/RF analyzer running incapacitance-measurement mode. Capacitance measurements were made atthree different frequencies (72, 117, and 174 MHz). The averagecapacitance values were corrected for fringe-field errors. The relativedielectric constant ∈_(r) of each sample was calculated using thecorrected capacitance, area, and thickness of the samples. The accuracyof the dielectric constant measurements was checked by measuring thedielectric constant of a known material, Plexiglas. The measured valuewas 2.2±0.2, while the literature reported value is 2.55±0.13.

Results

Thirty-three different silica aerogels were prepared to study theeffects of varying numerous parameters; e.g. using differentcombinations of APTES:TMOS ratio, type and percent epoxide moiety, andreaction conditions (time and temperature of reaction) for the sol gelhydrolysis reactions as described above. Parameters explored incross-linking amine-modified silica with polyfunctional epoxides were asfollows: the amine concentration on the silica backbone, the chemicalidentity and concentration of the epoxide in the mesopores, and finallythe reaction temperature and time. Due to the poor general solubility ofthe epoxides in methanol, THF was used as the solvent. (It is noted thatseveral of the previously-described wash steps could be eliminated, thusshortening the sol gel process significantly, if methanol were replacedby THF as the gellation solvent).

The amine concentration on the silica backbone was controlled by therelative ratio of APTES to TMOS in the sol. Thus, the APTESconcentration was evaluated from 0% up to 50% v/v in the total volume ofTMOS+APTES. Higher concentrations of APTES did not produce stable gels.For example, using a 3:1 v/v ratio of APTES to TMOS (75% APTES) producedopaque white, gelatinous (as opposed to rigid), and extremely fragilewet gels, while 100% substitution of APTES for TMOS results in sols thatfail to gel altogether. Hydrolysis of APTES is a much slower processthan hydrolysis of TMOS; thus, one explanation for the gelatinoustexture at 3:1 v/v ratio of APTES to TMOS (75% APTES) could be the factthat the amount of TMOS is too small to form stable gels. However,control experiments showed that keeping the relative ratio of APTES/TMOSto 1:3 v/v while reducing the total amount of APTES+TMOS in the solproduced gels even when the total amount of APTES+TMOS was reduced by 20times. Presumably, high concentrations of APTES in the sol may alsoresult in steric interferences to gellation as mentioned previously.Hence, while it was deemed reasonable that higher concentrations ofAPTES might yield the most sites for reaction with an epoxide, lowerconcentrations of APTES might produce a stronger silica framework tobuild upon.

As noted above, three different epoxides (di-, tri- andtetra-functional) were employed. Three different concentrations ofepoxides also were used, varying from 15% to 75% v/v of thecorresponding epoxide to THF solvent. Finally, effects of the reactiontime and the reaction temperature also were investigated. The reactiontemperature was constrained to a range between ˜50° C., below which thereaction is found empirically to be extremely slow, and ˜95° C., aboveexcessive solvent boil-off occurred. It was surmised, however, theinability to increase the reaction temperature indefinitely could becompensated with longer reaction times. Thus, the reaction time wasvaried from 16 to 72 h.

The density for all 33 samples was determined from their physicaldimensions and their weight. Surface area and pore diameters weredetermined by nitrogen adsorption porosimetry. Mechanical strength data(i.e., stress at break point and elastic modulus) were obtained by athree-point bend test method. These and other data are reported below inTable 1 for all 33 of the samples that were prepared.

TABLE 1 Physical characterization of epoxy cross-linked silica aerogelssurface average load APTES epoxy epoxy temp, density, area.^(c) poreforce, max stress, modulus, weight run percent type percent time, h ° C.g/cm³ m²/g diam.,^(d) A kg 10³ N/m² MPa loss, % 1 50 tetra 75 16.00 50.00.42 358 105 2.318 3.70 44.07 62 2 25 tri 15 44.00 72.5 0.42 443 1165.112 7.78 53.50 66 3 0 tetra 15 72.00 95.0 0.44 662 125 2.497 3.4633.33 58 4 50 di 75 16.00 95.0 0.30 462 114 0.990 1.33 13.30 60 5 25 tri45 44.00 72.5 0.48 350 134 5.820 8.77 72.64 64 6 25 tri 45 44.00 72.50.47 355 151 6.942 10.12 71.95 63 7 25 tri 45 44.00 95.0 0.51 280

143

11.448 16.93 87.93 68 8 50 di 75 72.00 50.0 0.32 446 186 0.978 1.3918.84 57 9 50 di 15 72.00 95.0 0.30 439 180 0.965 1.32 11.56 57 10 25tri 45 44.00 72.5 0.49 328 146 9.439 14.16 82.17 63 11 25 tri 75 44.0072.5 0.59 290 120 5.275 8.88 126.29 64 12 0 tri 45 44.00 72.5 0.36 267140 2.225 2.97 18.28 54 13 0 tetra 75 72.00 50.0 0.24 670

 83

a a a 14 14 50 di 15 16.00 50.0 0.28 493  86 0.661 0.06 14.55 47 15 25tri 45 44.00 72.5 0.49 309 141 5.473 8.04 74.67 63 16 25 tetra 45 44.0072.5 0.42 450 125 3.196 4.95 46.50 55 17 0 tetra 15 16.00 50.0 0.20 821112 a a a 10 18 25 tri 45 44.00 72.5 0.48 343 147 7.269 10.99 78.69 6219 50 tetra 15 16.00 95.0 0.32 464 132 1.466 2.00 17.66 59 20 50 tri 4544.00 72.5 0.40 278  91 5.750 7.47 34.98 70 21 0 di 75 72.00 95.0 0.24444 100 0.341 0.45 3.24 32 22 25 tri 45 16.00 72.5 0.44 345 156 5.1557.44 63.17 60 23 50 tetra 75 72.00 95.0 0.41 335

104

2.195 3.15 32.99 66 24 50 tetra 15 72.00 50.0 0.32 459  99 1.521 2.1819.43 56 25 0 di 15 16.00 95.0 0.26 784 175 a a a 13 26 25 tri 45 44.0072.5 0.47 312 151 7.775 11.28 71.46 63 27 25 tri 45 72.00 72.5 0.49 314147 12.950 19.05 80.08 64 28 25 tri 45 44.00 50.0 0.47 316

119

6.043 9.39 70.41 60 29 0 di 15 72.00 50.0 0.21 856  62 a a a 10 30 25 di45 44.00 72.5 0.33 546 145 1.855 2.79 24.35 45 31 0 tetra 75 16.00 95.00.29 423 115 0.440 0.67 6.94 28 32 28 tri 35 72.00 50.0 0.45 b b 7.2110.03 52.00 b 33 29 tri 35 72.00 50.0 0.48 b b 7.39 11.57 50.70 b^(a)Samples were too fragile for testing. ^(b)samples not tested.^(c)Surface area of non-cross-linked aerogels: 25% APTES = 739 m²/g: 50%APTES = 622 m²/g. ^(d)Pore diameter of non-cross-linked aerogels: 25%APTES = 117 A. 50% APTES = 89 A.

indicates data missing or illegible when filed

“Weight loss %” in Table 1 refers to the total weight loss fromthermogravimetric analysis, used here to determined the total amount oforganic material in the cross-linked aerogels. Note for instance, whenno APTES is used the amount of weight loss is 10-30%, whereas with25-50% APTES, weight loss is usually around 60%. This indicated thatepoxy reacts only slightly with Si—OH on the surface, but much moreefficiently with amine.

Typical epoxy cross-linked silica aerogels are shown in FIG. 3, alongwith a sample prepared with a 25% APTES to (APTES+TMOS) volume ratio andno epoxy cross-linking (sample furthest to the left). As seen in FIG. 3,the epoxy cross-linked aerogels generally were translucent. Theyellowish coloration observed in both the un-cross-linked and thecross-linked samples comes from a trace amount of base-catalyzedpolymerization of acetone, the solvent used in the final washes beforesupercritical fluid drying. For NH₄OH-catalyzed gels, this colorationcan be avoided by removing the base with alcohol washes beforeintroducing acetone. For APTES-containing gels, this is obviously notpossible because the base is an integral part of the gel backbone.

Microscopically, native APTES/TMOS-containing gels were similar totypical base-catalyzed gels produced from TMOS without APTES. FIG. 4illustrates SEM micrographs of four of the aerogels produced accordingto the above methods. Specifically, FIGS. 4 a-4 d are micrographs ofsilica aerogels, all produced from a 1:3 APTES:TMOS ratio but withdifferent (or no) epoxide species used for polymer cross-linking.Specifically, 4 a, 4 b, 4 c and 4 d show aerogels that were prepared viacross-linking, respectively, with no epoxy cross linking (4 a), with thedifunctional epoxide (4 b), with the trifunctional epoxide (4 c), andwith the tetrafunctional epoxide (4 d). From the micrographs in FIG. 4,it is possible to distinguish primary particles clustered together toform larger secondary particles, which in turn form a “pearl-necklace”type of structure with large voids (mesopores), comparable to theschematic structure illustrated in FIG. 1. The epoxy cross-linkedsamples look similar to one another, irrespective to the chemicalidentity of the epoxide employed for cross-linking (FIGS. 4 b-d).

It was also observed, e.g. from the micrographs in FIG. 4, that thepolymeric cross-linking structure was manifested as an accumulation ofpolymeric material that followed the surfaces of the secondary particlesto produce a conformal coating over those particles and substantiallyobscured all fine particle definition between secondary particles. Incontrast, the mesoporous networks in these aerogels was largelyunaffected by the polymer cross-linking structure, and as seen in FIG. 4remained visible in all three cross-linked aerogel samples (4 b-4 d)shown in that figure. Thus, the cross-linked epoxy polymeric networkproduced essentially a polymeric coating over a silica backbone composedof the interconnected secondary particle strands in native silica, butdid not extend substantially into the mesoporous space defined in thesolid network between the secondary particle strands. Thus, thepolymeric network effectively reinforced (as will be seen) the solidnetwork without significantly compromising the total mesoporosity of thesilica macrostructure.

Density and mechanical strength data for all samples from Table 1 areplotted in FIGS. 5-7 versus the run order and distinguished by epoxytype. In FIGS. 5-7, circles represent difunctional epoxidecross-linking, squares represent trifunctional epoxide cross-linking,and triangles represent tetrafunctional epoxide cross-linking. Thesamples were prepared and tested in random order, so the time-seriesplots in FIGS. 5-7 display a random distribution of values, indicatingthat no time-dependent errors (e.g., aging of monomers, temperaturedrift, etc.) were present. For comparison, lines in the graphs of eachof FIGS. 5-7 show typical values for density, stress at breakpoint, andmodulus for native aerogels (i.e., non-APTES and nonepoxy-modified).Thus, the data in these figures illustrate that while the density ofepoxy-cross-linked aerogels is modestly (at most 2-3 times) greater thanthe density of native silica aerogels, both stress at breakpoint andmodulus were dramatically increased by as much as two orders ofmagnitude. So significant an increase in mechanical strength (100 timesor greater) with only a modest increase in overall material density wasan extremely surprising and unexpected result. Preferably, polymercross-linked ceramic oxide particle networks according to the inventionwill have not more than 10, preferably 8, preferably 5, preferably 4,most preferably 2 or 3, times the bulk density of the correspondingnative, uncross-linked aerogel, but at least 10 times, and preferably atleast 100 times, greater elastic modulus and ultimate strength (stressat failure or break).

In addition, the data in FIGS. 5-7 illustrate that the increases in allthree of the measured physical properties was most striking for thetrifunctional epoxy. The data also show that when the trifunctionalepoxide was used for cross-linking, more polymeric material wasincorporated into the aerogel. This was believed indicative of thedifference in reactivity between the epoxide linked through an oxygen(as in the trifunctional epoxide employed herein) versus those linkedthrough nitrogen to a central aromatic core. If the reactivity of thesetwo kinds of epoxides were the same, then a larger molecule such astetra-epoxy would have shown higher density increases, especially atlower APTES concentrations where amine sites might be too far apart toallow multiple epoxy moieties to bridge across two amines effectively.More specifically, from the data there appears to be a synergisticeffect on density between the number of reactive amine sites (percentAPTES) on the surfaces of the secondary silica particles and the epoxyconcentration. At 0% APTES, increasing epoxy concentration in the THFbath had little effect on the density (since reactivity of any epoxidewith the —OH groups on the surfaces of a native aerogel was expected tobe low), while at higher concentrations of APTES, increasing the epoxyconcentration from 15 to 75% caused the density to double. It is notedthat density reached a maximum at around 30% APTES, above which densityleveled off. This suggests that at higher concentrations of APTES, asthe surfaces of the aerogel particles become fairly crowded with aminesites, more epoxy molecules begin to bridge multiple amine sites. Butafter a certain APTES concentration, epoxy molecules react with only oneamine site rather than bridging multiple sites, adding weight but noadditional cross-linking.

Conversely, the reaction time versus temperature data in Table 1 alsoillustrate that these variables have only a slight effect on densitywhen epoxy and APTES concentration were held at their optimum conditionsfor strength. At lower concentrations of APTES, both had a morepronounced effect on density.

Average pore diameter and specific surface area data in Table 1 weremeasured using the Brunauer-Emmet-Teller (BET) method. In general, bycomparing the BET surface area with density for the aerogels, it wasconcluded that samples with higher density have lower surface area. Thetri-epoxy samples, which had the highest densities, were the lowest insurface area, followed by tetra and di-epoxy samples. Also, surface areagenerally decreased with increasing epoxy concentration, APTESconcentration, time, and temperature, while density increased under thesame conditions. This inverse relationship between surface area anddensity was attributed to the fact that accumulated cross-linker clogschannels to the micropores, making those surfaces inaccessible fornitrogen adsorption.

Stresses at breakpoint and moduli for the aerogel samples both in Table1 and FIGS. 5-7 were calculated from stress-strain curves obtained fromthree-point bend test results for two randomly selected samples ofvariable density. Stronger samples were found also to be stiffer (theyrequired a higher force for a given amount of deformation) and tougher(larger area underneath the stress-strain curve; i.e., they can storemore energy at the breakpoint). Interestingly, unlike the isocyanatecross-linked aerogels described in the above-mentioned U.S. patentapplication publication, the present epoxide cross-linked aerogelsexhibited almost completely elastic behavior, having a substantiallylinear stress-strain relationship all the way to the point of break orfailure (ultimate strength). This is probably attributed to the factthat polymer is actually built up in the case of the isocyanatecross-linked aerogels, where isocyanate chains generally are long andhighly branched, producing a thick coating on the secondary particlestrands, but not in the case of epoxy linkages which tend to berelatively short, such as just several monomers long. This at leastsuggests a design criterion for producing elastic versus inelasticaerogels based on selection of suitable cross-linking architectures.

From these experiments, it is evident that incorporation of epoxyincreases strength of the aerogels over and above what is to be expectedby simple densification, while relatively large surface areas and notmuch larger pore diameters imply that mesoporosity has been retained.Therefore, it is reasonable to expect that polymeric reinforcement ofaerogels through cross-linking should not affect the properties thatrender aerogels attractive materials for practical applications.

¹³C and ²⁹Si spectrographic analysis of the prepared samples furtherindicated that at best two epoxy groups per epoxide molecule reactedwith the aerogel surface (whether APTES or siloxy group), regardless ofwhether there were two, three or four epoxy moieties available permolecule. Furthermore, a higher percentage of APTES in the aerogel didnot necessarily provide more cross-linking between APTES sites. Rather,more epoxy was bonded to only one APTES, especially evident in the caseof the tri-epoxy when the maximum amount of APTES was used. Furthermore,spectrographic data also suggested that APTES incorporated into thesilica network was located primarily at the surfaces of the secondaryparticles. As the APTES concentration increased relative to TMOS in thehydrolysis reactions for preparing the gels, the corresponding Si NMRpeak for the APTES group incorporated into the silica network increasedin substantially linear relationship (integrated area under the peak)compared to a decrease in the peak assigned to the hydroxyl-containingSi groups present on the secondary particle surfaces. This suggestedthat APTES amine groups were located primarily at the surfaces ofsecondary silica particles, thereby replacing hydroxyl groups on thosesurfaces.

Additional information regarding the present experiments can be found inthe inventors' publication, Mary Ann B. Meador et al., “Cross-linkingAmine-Modified Silica Aerogels with Epoxies: Mechanically StrongLightweight Porous Materials,” Chem. Mater., vol. 17, pp. 1085-1098,2005, which is incorporated herein by reference in its entirety.

Example 2 Isocyanate Cross-Linked Silica Aerogels from Amine-DecoratedSilica Network

Experiments also were conducted to demonstrate the efficacy ofcross-linking amine-decorated silica (TMOS-APTES copolymerization asdiscussed above) using polyisocyanates to produce a polymercross-linking architecture. To date, cross-linking withisocyanate-derived chemistry utilized only surface silanols, whichcomprise reaction sites for the diisocyanate. See U.S. patentapplication publication No. 2004/0132846, incorporated herein.

Because hydrolysis of APTES is slower than hydrolysis of TMOS, inaddition to other factors described hereinabove, the amine functionalitywas found to be mostly positioned on the surfaces of the secondarysilica particles where it was readily available for cross-linking.Through reaction with both the surface silanols and amines, a conformalpolymer coating was attached to the surface of the string of pearlsstructure of the secondary particle strands as both carbamate (urethane)and polyurea.

Using a systematic approach, the concentration of the co-polymerizedsilanes (TMOS and APTES as described previously) in the sol gel, theconcentration of diisocyanate the gels were exposed to, and the reactiontemperature were varied. The resulting gels were dried by supercriticalcarbon dioxide extraction. Following this, the maximum stress at failureand modulus were measured using a three-point flexural bending test, andbulk density was determined, also similarly as above. From these data,significant effects of the variables on the measured properties wereascertained, which effects in turn are useful to predict properties ofmonoliths prepared using other combinations of polymer cross-linkingarchitecture with silica.

Twenty-eight different aerogel monoliths plus two repeats were made assummarized in Table 2.

TABLE 2 Physical properties for isocyanate cross-linked, amine-decoratedsilica aerogels Total silane, Monomer Stress at % conc., % Cross- Bulkfailure, v/v in w/w in linking T, density, N/m² × Modulus, Sample CH₃CNCH₃CN ° C. g/cm³ 10⁴ MPa 1 30.0 0.0 25 0.136 1.32 6.91 2 30.0 13 250.312 466.53 15.1 3 30.0 6.8 25 0.270 25.33 11.2 4 30.0 2.3 25 0.2258.65 8.97 5 30.0 13 71 0.449 158.60 46.7 6 30.0 6.8 71 0.351 50.30 25.47 30.0 2.3 71 0.283 37.96 15.8 8 17.6 0.0 25 0.083 2.45 2.61 9 17.6 1325 0.190 7.95 4.58 10 17.6 6.8 25 0.179 7.54 4.27 11 17.6 2.3 25 0.1615.56 3.86 12 17.6 13 71 0.347 51.52 28.8 13 17.6 6.8 71 0.316 40.04 25.314 17.6 2.3 71 0.234 14.41 10.7 15 7.16 0.0 25 0.042 0.67 1.01 16 7.1613 25 0.090 8.29 0.66 17 7.16 6.8 25 0.083 6.92 0.44 18 7.16 2.3 250.075 5.71 0.41 19 7.16 13 71 0.155 7.74 3.44 20 7.16 6.8 71 0.143 8.543.69 21 7.16 2.3 71 0.106 6.37 1.19 22 4.10 0.0 25 0.026 0.09 0.10 234.10 13 25 0.043 2.22 0.12 24 4.10 6.8 25 0.038 1.68 0.10 25 4.10 2.3 250.036 1.21 0.10 26 4.10 13 71 0.072 3.40 0.36 27 4.10 6.8 71 0.059 2.750.26 28 4.10 2.3 71 0.057 2.11 0.21 29 30.0 13 25 0.207 162.58 5.25 307.16 2.3 25 0.046 4.10 0.14

As seen from table 2, three preparation conditions were systematicallyvaried: four different concentrations each of total silane (APTES+TMOS)and diisocyanate, and two different polymerization temperatures. Initialtotal silane concentration (APTES+TMOS in a 1 to 3 v/v ratio) was variedfrom 4.1%-30% by volume in acetonitrile (CH₃CN) to determine the densityof the underlying silica aerogel, while the amount of diisocyanate wasvaried from 0% w/w to 13% w/w in CH₃CN and the cure temperature from 25°C. to 71° C. to determine the degree of cross-linking.

A typical procedure required the mixing of two solutions similarly as inEXAMPLE 1, one containing the silane precursors in acetonitrile solventat the prescribed concentration as shown in Table 2, the othercontaining 25% water by volume in acetonitrile. Also as before, theamine-rich APTES eliminated the need for additional base catalysis, andit was necessary to cool the two solutions, this time in anisopropanol/dry ice bath, before mixing together to prevent prematuregellation. The solutions were then shaken vigorously and poured intoappropriate polypropylene molds. Gellation occurred in anywhere from oneminute (30% silane solutions), to a few hours (4.1% solutions). Theresulting sol gels were washed a total of four times at 12 hoursintervals to remove excess water from the mesopores, leaving only themore strongly adsorbed water on the surface of silica. The gels werethen placed in a diisocyanate (Desmoder N3200, a 1,6 hexamethylenediisocyanate-based oligomer) bath of varying concentrations (as shown inTable 2) and allowed to equilibrate until no concentration gradientswere observed (usually 24 hours). Afterwards, the monomer solution wasdecanted, replaced with fresh acetonitrile, and allowed to react for 72hours either at room temperature or in a 71° C. oven. At the end of theperiod, oven-cured gels were cooled to room temperature, and the solventwas replaced four times in 24 hour intervals to remove any un-reactedmonomer from the mesopores of the wet gels.

The resulting sol gels then were dried using supercritical CO₂extraction as described herein to give the resulting polymercross-linked aerogel monoliths, and their properties were measured asreported in table 2 above.

The highest density cross-linked aerogel exhibited the highest maximumstress at failure and the highest modulus. Stress at failure for theseaerogels was 350 times higher than that of the correspondingnon-cross-linked aerogels, while density only increased by a factor oftwo. The lowest density cross-linked aerogels, which boasted a nominaldensity of 0.036 g/cm³, still exhibited a forty-fold increase in stressat break over the corresponding uncross-linked aerogels with the sametwo-fold increase in density.

It was also determined from the above data, through a statisticalmodeling procedure, that the effect of increasing diisocyanateconcentration strongly increased the maximum stress at failure, whileits effect on density and modulus was modest, though still statisticallysignificant. Density, maximum stress at failure and modulus allincreased with increasing silane concentration as well. For maximumstress, there was also a significant synergistic effect between silaneand diisocyanate concentration. In other words, there was a greaterincrease in maximum stress with increasing silane concentration when thediisocyanate concentration was also high.

Temperature also had a pronounced effect on all three measuredresponses, density, maximum stress and modulus. All three of these werehigher for all combinations of both diisocyanate and total silaneconcentration. Furthermore, solid C3 CP-MAS NMR spectra of selectedmonoliths revealed approximately twice the amount of polymerincorporated from the 71° C. heated samples compared to thosepolymerized at room temperature, as evidenced by integrating one of theAPTES methylene peaks against the peaks for the nominally 12 methylenesper repeat unit of the diisocyanate oligomer. This data suggested thatalthough room temperature may provide enough energy for the reactionbetween the diisocyanate and the APTES amines on the surface of thesilica network in the sol gel, elevated temperatures facilitate thereaction to a greater extent. Given that reaction between thediisocyanate and surface hydroxyl groups requires elevated temperatures,and no room temperature cross-linking was observed fornon-amine-modified silica surfaces, it was concluded that roomtemperature cross-linking favors attachment of the diisocyanate throughreaction with surface amines. Concurrently, cross-linking at elevatedtemperatures promotes reaction with both surface amines and residualhydroxyls.

To better illustrate how the material's nano-structure relates to themacroscopic properties observed, nitrogen adsorption data were analyzedfor surface area and average pore diameter by the Brunauer-Emmet-Teller(BET) method for selected samples from Table 2. Non-cross-linkedaerogels from this study typically had a surface area of 600-700 m²/gdepending on the concentration of silanes in the starting sol, whilesurface areas for the diisocyanate cross-linked aerogels were in therange of 200 to 300 m²/g for monoliths cross-linked at 71° C., and 300to 400 m²/g for those cross-linked at room temperature. In general, asthe amount of diisocyanate incorporated in the aerogel increased (eitherthrough increasing diisocyanate concentration or higher curetemperature), the surface area dropped only by a factor of 2. Theaverage pore diameter followed a similar trend as surface area asuncross-linked samples exhibited measured pore diameters of around 8.1Å, while cross-linked samples fell to between 6.4 to 7.5 Å.

Interestingly, as the amount of underlying silica in the polymercross-linked aerogels was decreased, the monoliths exhibit a change inmicrostructure. At high densities such as at 450 mg/cc as shown in FIG.8 a, the secondary particles resembled a cloud-like configuration withmultiple attachments to other secondary particles. In contrast, FIG. 8 bshows the microstructure of a lower density aerogel, 36 mg/cc, having amore fibrous-like, elongated particle arrangement. It also appears thatthe secondary particles are ill-defined in the lower-density polymercross-linked aerogel, strongly indicating a thicker conformal polymercoating over the secondary particles.

Though the combination of pore diameter and surface area together givesan accurate representation of the material's nanostructure, the truedifferences in overall nano-porosity are quantified via Eq. 11 whereρ_(b) is the measured bulk density and ρ_(s) is the measured skeletaldensity of the sample aerogels:

$\begin{matrix}{{{Porosity}\mspace{14mu} (\%)} = {\frac{{1/\rho_{b}} - {1/\rho_{s}}}{1/\rho_{b}} \times 100}} & (11)\end{matrix}$

Thus, one of the highest density native aerogels produced in this study(ρ_(b)=0.136 g/cm³) has a calculated porosity of 91.9%. When this samplewas cross-linked using the highest monomer concentration in thisexperiment, the resulting monolith (ρ_(b=0.312) g/cm³) still remainedover 75% porous. (For example, consider the dark spaces in FIG. 8 a). Atthe same time, the porosity of the lowest density native aerogelproduced in this study (ρ_(b=0.026) g/cm³) was extremely high (97.5%).When coated with enough cross-linked polymer to produce the lowestdensity cross-linked aerogel (ρ_(b=0.036) g/cm³), the resulting polymercross-linked aerogel monolith retained an astonishing 95.1% empty space(dark spaces in FIG. 8 b).

Concomitant with this change in morphology, the aerogels also were foundto become more flexible via three-point bending experiments. Thisproperty was observed in those cross-linked aerogels with densities lessthan 0.1 g/cm³, and is attributed to fewer attachment points betweennanoparticles in the lower density nanostructure. Evidently, as thesilica structure becomes more open and fibrous, the properties of thepolymer cross-links begin to emerge, and in some cases, dominate thoseof the rigid silica support framework.

Flexibility is a property that aerogels have not previously exhibited,and thus it is indicative that at very low silica content, the aerogeltakes on more properties of the polymer cross-linking structure. The useof other types of polymers (toughened epoxies, rubbers, etc.) shouldintroduce even more flexibility into the system. Thus it will beillustrated that an empirical model can be established to tailoraerogels with a particular set of properties for a wide variety ofapplications, by quantifying a broader range of processing conditions(for example, varying the amount of water, catalyst, washings, reactiontime, etc.) for a host of other properties such as thermal conductivityand compressive strength, in addition to density, flexural modulus andstress at failure.

Example 3 Polystyrene Cross-Linked Silica Aerogels from Amine-DecoratedSilica Network

In another experiment, styrene-containing species were linked to theterminal amines of amine-decorated secondary silica particles made bycopolymerization of TMOS with APTES as described above. In theseexperiments, the chloride on 4-vinylbenzyl chloride was reacted with theterminal amines to attach the styrene moiety to the amine groups,generally of the form

Then the terminal styrene moieties now attached to the secondaryparticle surfaces were reacted with styrene-containing monomers, andpolymerized to produce polystyrene polymeric cross-links according tothe following methodology. It will be understood that in addition to thesecondary amine structure illustrated above, tertiary amine structuresalso are possible, particularly in the presence of excessstyrene-containing species. Quaternary amine structures also may bepossible, in which case the N atom will have a net positive charge.However this is unlikely at least due to steric considerations.

First, the wet sol gels were prepared similarly as already described.All sol gels were prepared by mixing together two solutions; solution Awhich contained any silanes and half of the solvent, in this casemethanol, and solution B which contained the other half of the solvent,water and any catalyst, if used. Wet gels withoutaminopropyltriethoxysilane or APTES (as a control) were prepared bymixing together the following mole ratios of TMOS (1 mol), deionizedwater (3 mol) and methanol (8.5 mol). Catalysis of all gels was providedby adding 6.0E-06 mol of ammonium hydroxide (30% v/v) for every mol ofTMOS used in the recipe. Ammonium hydroxide was added to all of the gelrecipes for consistency, even though gels including APTES did notrequire separate catalysis as explained above. Wet gels containing APTESwere prepared by adding either 25% v/v or 50% v/v APTES to TMOS. Thereaction to form the APTES-containing sol gels had to be slowed down topermit sufficient time to pour the reactive sol into appropriate moldsafter combining all reactants. This was achieved via cooling in a dryice-acetone bath prior to mixing solutions A and B for each sol.

Immediately after mixing, all sol-gel solutions were poured intocylindrical molds approximately 1 cm in diameter and four centimetershigh to form the desired monoliths for testing. The molds were thensealed with Parafilm and the gels allowed to age for 48 hours. The solgels were then carefully removed from the molds and were washedapproximately every 12 hours with methanol for a total of four times toremove residual catalyst and unreacted TMOS or APTES. The wet gels werethen washed twice with tetrahydrofuran and twice with anhydroustetrahydrofuran again every 12 hours.

Then, the APTES modified silica sol gels were placed in a THF solutionof 4-vinylbenzyl chloride (13% v/v) and allowed to react at 45° C. for72 hours to attach the terminal styrene moieties to the amine functionalgroups at the surface of the secondary silica particles as illustratedabove. The resulting sol gels were then washed once with fresh THF toremove unreacted species, and four times with chlorobenzene for 6 hoursintervals to obtain the final styrene-functionalized wet gels.

The styrene-functionalized wet gels were placed into a solution ofstyrene-containing monomers and AIBN (radical initiator) inchlorobenzene solvent. Three styrene-containing monomers were used inthis experiment: styrene, 2,3,4,5,6-pentafluoro styrene and4-vinylbenzyl chloride. Each wet gel was placed in 20 mL of one of thefollowing solutions to obtain a cross-linked wet gel based on thedesired monomeric styrene-containing species:

-   -   a) Styrene monomer (75 mL) and AIBN (3 g) were added into 225 mL        of chlorobenzene to obtain a clear solution.    -   b) Styrene monomer (100 mL), 4-vinylbenzyl chloride monomer (50        mL) and AIBN (3 g) were added into 150 mL of chlorobenzene to        obtain a clear solution.    -   c) 2,3,4,5,6-pentafluoro styrene monomer (18 mL) and AIBN        (0.2 g) were added into 22 mL of chlorobenzene to obtain a clear        solution.

The gels were then held at 10° C. for 18 hours followed bypolymerization at 75° C. for varying time periods in the respectivemonomeric solutions. Gel permeation chromatography (GPC) studies wereperformed on the resulting solutions to ascertain the degree ofpolymerization for each of the gels. The resulting cross-linked sol gelswere washed once with chlorobenzene, once with (25:75)acetone:chlorobenzene, once with (50:50) acetone:chlorobenzene, oncewith (75:25) acetone:chlorobenzene, and twice with acetone by keepingthe gels for 6 hours in each of the mentioned solvent systems. Finally,all of the sol gels were dried to produce the corresponding polystyrenecross-linked silica aerogels via the above-described supercritical CO₂exchange and extraction method.

From these experiments, it was observed that better results wereobtained from 25% APTES containing gels compared to gels containing 50%APTES (based on APTES+TMOS). Because an excess of 4-vinylbenzyl chloridewas used to provide the styrene functionality to the silica sol gels, amixture of secondary and tertiary amines containing styrene units werebelieved to result on the secondary particle surfaces. During thepolymerization step, it was believed that the surface-bound styrene andfree monomers in the solution reacted together, resulting in bothbinding of different secondary silica particles (e.g. in the neckregions between adjacent secondary particles) and free-end polymergrowth from the surface-bound styrenes. The initial monomerconcentration and the time for polymerization were used to control thedegree of cross-linkage and the final aerogel density, as shown byexperimental data. As a control, the 0% APTES gels were subjected to thesame styrene functionalization and polymerization conditions. Followingthese steps, no significant polymer loading was observed on the 0% APTESgel, which demonstrated the importance of providing the surface-boundamine groups. Some experimental data from selected styrene cross-linkedaerogels appears below in Tables 3 and 4.

TABLE 3 Physical properties of 4-vinylbenzyl chloride/styrenecross-linked aerogels Aerogel BET Area, Type, m²/g APTES % Diameter,Density, Stress, (Av. Pore Modulus, time cm gr/cm³ Mpa diam., nm) Mpa25% 3 hr 0.896 0.618 0.251 243.5 (11.0) 132.82 25% 6 hr 0.889 ± 0.0020.635 ± 0.005 0.157 ± 0.016 226.0 (11.2) 125.55 ± 3.07 25% 24 hr 0.885 ±0.002 0.717 ± 0.006 0.247 ± 0.005 180.4 (11.5) 161.44 ± 10.65 50% 3 hr0.976 ± 0.005 0.505 ± 0.003 0.201 ± 0.041 244.1 (16.3)  60.79 ± 2.00 50%6 hr 0.951 ± 0.004 0.530 ± 0.005 0.111 ± 0.010 262.0 (13.9)  71.15 ±0.27 50% 24 hr 0.980 ± 0.015 0.546 ± 0.017 0.081 ± 0.026 228.6 (14.9) 73.50 ± 2.27

TABLE 4 Physical properties of styrene cross-linked aerogels and2,3,4,5,6-pentafluoro styrene cross-linked aerogel Aerogel BET Area,Type, m²/g APTES % Diameter, Density, Stress, (Av. Pore Modulus, time cmgr/cm3 Mpa diam., nm) Mpa 25% 3 hr 0.881 ± 0.011 0.457 ± 0.017 0.093 ±0.014 392.8 (11.2) 38.82 ± 0.85 25% 6 hr 0.887 ± 0.002 0.477 ± 0.0060.118 ± 0.014 368.3 (11.6) 62.57 ± 10.83 50% 3 hr 0.941 ± 0.020 0.413 ±0.013 0.068 ± 0.012 390.1 (14.6) 26.75 ± 4.11 50% 6 hr 0.967 ± 0.0040.419 ± 0.010 0.059 ± 0.002 346.0 (12.5) 26.16 ± 0.61 50% 24 hr 0.9630.768 0.387 212.8 (11.4) 94.25

Example 4 Extrapolation of Disclosed Methods to Other Cross-Linking andCeramic Oxide Species

It should be apparent, as noted above, that a wide variety ofpolymer-reinforced ceramic oxide aerogels can be prepared based ondifferent combinations of cross-linking architecture (polystyrene,epoxy, isocyanate, etc.). While these three have been actually preparedas described in the foregoing examples, the variety and distinctionsamong the involved chemistries indicates that other cross-linkingarchitectures also could be employed by attaching the appropriate baseor monomeric species to support the desired polymeric architecture ontothe surface of the ceramic oxide. In the case of silica, it has beenshown that APTES, a functionalized silica precursor species, can becopolymerized with TMOS, a non-functionalized silica precursor species,via a hydrolysis reaction that will remove all of the alkoxy groups fromthe silicon atoms in both TMOS and APTES (because they are all attachedvia hydrolysable bonds), but not the aminopropyl group attached to theAPTES silicon (because it is attached via a non-hydrolysable bond). Totake advantage of the analogous sol gel chemistry to produce ceramicoxide networks based on other, non-silica metallic or nonmetallic atoms,one need prepare the analogous functionalized and unfunctionalizedprecursor species. Alternatively, the ceramic oxide mesoporous networkcan be decorated with appropriate functional groups by post-gellationtreatment with molecules having the desired functional groups that willsupport cross-linking via the desired chemistry, but also having anotherreactive site that can be used to attach the molecule (having thereactive species) to the ceramic oxide network. For example, themesoporous surfaces of a metallic or semimetallic ceramic oxide networkcan be decorated with amines by post-gellation treatment with APTES, orthey can be modified with styrene by post-gellation treatment withp-trimethoxysilyl styrene.

Example 5 Isocyanate Cross-Linked Ceramic Aerogels from NativeSurface-Bound Hydroxyl Groups for Numerous Ceramic Oxides

To demonstrate the efficacy of cross-linking a variety of ceramic oxidespecies, over thirty different species of ceramic oxides werecross-linked via an isocyanate cross-linking architecture analogous tothat described in Application Publication No. 2004/0132846 for silica.Specifically, surface-bound isocyanate groups were introduced in therespective ceramic oxide networks via reaction of a diisocyanate withthe surface-bound hydroxyl groups natively present on ceramic oxidesurfaces to produce carbamate (urethane) linkages. Then, the freeisocyanate groups of the surface-bound diisocyanates reacted withgellation or coordination water remaining adsorbed on the surface of theceramic oxide nanoparticles to form amines, which reacted with otherdiisocyanate monomers in the pore-filling solvent to produce across-linked polymer network of polyurea chains anchored to the ceramicoxide network surfaces via urethane linkages.

Polyurea cross-linked (using diisocyanates) ceramic oxide aerogels havebeen successfully prepared from over 30 different metallic elements.Aerogels from both the non-rare earth and rare earth transition metalswere prepared. These included but were not limited to aluminum,chromium, dysprosium, erbium, europium, gadolinium, gallium, hafnium,holmium, indium, lanthanum, lutetium, manganese, neodymium, niobium,praseodymium, ruthenium, samarium, scandium, tantalum, tellurium,terbium, thulium, titanium, tungsten, uranium, vanadium, ytterbium,yttrium and zirconium. All of these gels except for those based ontitanium, vanadium, and manganese were prepared using a non-hydrolyticroute according to the general procedure described by: (a) T. M.Tillotson, W. E. Sunderland, I. M. Thomas, L. W. Hrubesh Sol-Gel Scienceand Technology 1994, Vol 1, p 241; (b) A. E. Gash, T. M. Tillotson, J.H. Satcher Jr., L. W. Hrubesh and R. L. Simpson Journal ofNon-Crystalline Solids 2001, Vol 285, pp 22-28; (c) A. E. Gash, T. M.Tillotson, J. H. Satcher Jr., J. F. Poco, L. W. Hrubesh and R. L.Simpson Chemistry of Materials 2001, Vol. 13, pp 999-1007. In all cases,either the hydrated metal chloride or the anhydrous metal chloride wasdissolved into ethanol. Typically, the target concentration of the metalsalt was 0.35 mol/L, while the volume of the ethanolic solution was 20mL. For the anhydrous metal chloride salts, sufficient water was addedto the solution to result in a total of six to seven moles of water foreach mole of metal. Once the metal salt was completely dissolved inethanol, a proton scavenger, epichlorohydrin, propylene oxide ortrimethylene oxide was added to the solution in a 10 molar excess overthe metal salt. The mole ratio of proton scavenger to metal was 10 to 1.The solution was poured into cylindrical molds and gellation wastypically observed within 10-20 min. In some instances, however, (e.g.,indium oxide gels) gellation can take up to 3 days. Gels were left toage for at least 24 hours. Subsequently gels were pushed gently out ofthe molds into vials filled with ethanol. Ethanol was changed anotherthree times every 24 hours, and then the bathing solvent was changedeither to acetone or acetonitrile and was changed another 4 times every24 hours. At that point, wet gels were ready for polymer cross-linking.

Manganese oxide wet gels were prepared using the method described by J.W. Long, K. E. Swindler-Lyins, R. M. Stroud and D. R. Rolison,Electrochemical and Solid-State Letters 2000, Vol. 3, pp 453-456.Typically, potassium permanganate and fumaric acid dissolved in water ina 3:1 molar ratio were left to react in an open flask to vent carbondioxide produced by the reaction. After a few minutes the solution waspoured into molds where it was left to gel and age for at least 24hours. Gels were washed multiple times with water, an aqueous sulfuricacid solution, water again and finally with acetone. At that point theywere ready for polymer cross-linking as described below.

Titanium and vanadium aerogels were prepared from the correspondingalkoxides via sol-gel chemistry analogous to the sol gel chemistry usedto prepare the silica aerogels described above. In the case of titanium,the reaction rate was controlled with hydrochloric or nitric acid. Forvanadium, no catalyst was required and gellation was carried out inacetone-containing water solution following the general proceduresdescribed by F. Chaput, B. Dunn, P. F. Salloux Journal ofNon-Crystalline Solids 1995, Vol. 188, pp 11-18. An importantmodification was the use of vanadyl propoxide (VO(O-n-propyl)₃) ratherthan vanadyl triisopropoxide (VO(O-isopropyl)₃), because it was foundthat crack-free monoliths are mostly produced by the former precursor. Anumber of these gellation reactions were quite exothermic and tookseconds to gel. To slow down the rate of the reaction in order to allowtime for pouring the gels into the molds, solutions were cooled to −78°C. in a dry ice/acetone bath. After gellation, vanadium oxide gels wereaged for at least 24 hours, and subsequently were pushed into containersfilled with acetone or acetonitrile and were washed 4 times once every24 hours. After supercritical drying from carbon dioxide, many of thenative (not polymer cross-linked) gels were very delicate. Some were sofragile that they were difficult to handle, particularly those based ontitanium and lanthanum.

Samples of each of the corresponding wet gels also were cross-linkedusing Desmodur N3200; a conventional diisocyanate available from Bayer.This same cross-linking reaction had already been demonstrated toincrease the strength of silica aerogels prepared fromtetramethylorthosilicate as shown in Publication No. 2004/0132846. Tocross-link each of these metal oxide gels, each gel was washed fourtimes with the solvent used to prepare the wet gel and then washed fourtimes with either acetone or acetonitrile, the solvent used forcross-linking. The resulting wet gels were then placed into adiisocyanate solution and allowed to equilibrate for 24 h. Unlike thesilica aerogels, which required heat to facilitate cross-linking, mostof the other metal and semimetal oxide wet gels started reacting withthe diisocyanates at room temperature.

Material properties of thirteen native aerogels based on rare earthelements, and their corresponding diisocyanate cross-linked aerogels,are reported below in Table 5. The aerogels in Table 5 were ˜4 cm longmonoliths having the reported diameters. Also reported in Table 5 areproperties for some gels, referred to as “Xerogels,” which are native,uncross-linked aerogels that were not supercritically dried. The forceat rupture data provided in Table 5 were obtained through a three-pointflexural bending test similarly as described above in EXAMPLE 1.

TABLE 5 Selected comparative physical property data for native anddiisocyanate cross-linked ceramic oxide aerogels Elemental AnalysisSuscep- Metallic Aerogel (% w/w) Diameter Density (g/cm3) BET SurfaceForce at Modulus tibility Element Sample Metal Co₃ ²⁻ Cl_(T) (Cl_(W))(mm) Bulk Skeletal Area (m²/g) rupture (kg)₁ (MPa)₁ (units × 10⁶) ScNative 36.61 4.4 — 6.12 ± 0.28 0.12 ± 0.03 — 592 (7.29) — — −0.6 ± 0.3X-linked 8.2 3.6 4.4 7.98 ± 0.53 0.21 ± 0.04 1.15 ± 0.01 242 (6.89) 1.53± 0.14  6.2 ± 3.1 −0.6 ± 0.1 Xerogel 34.77 15.15 — 2.69 ± 0.06 1.20 ±0.03 — 478 (2.31) — — −0.2 ± 0.0 Y Native 45.52 12.76 7.6 (6.8) 8.57 ±0.14 0.12 ± 0.01 2.39 ± 0.19 528 (7.41) — — −0.3 ± 0.1 X-linked 13 5.25— 8.33 ± 0.02 0.36 ± 0.01 1.36 ± 0.00 144 (9.24)  5.67 22   −0.4 ± 0.1Xerogel 46.6 2.6 8.1 3.31 ± 0.10 1.49 ± 0.09 — 431 (2.74) — — −0.2 ± 0.0La Native 45.4 — 8.6 (6.1) — — — 128 (8.34) — — −0.1 ± 0.0 X-linked 311.69 — 9.84 ± 0.68 0.13 ± 0.02 1.51 ± 0.02  70 (8.32) 0.25 ± 0.05 16.0 ±0.9 −0.4 ± 0.1 Xerogel 59.3 — 14.2 (7.7)  — — — 208 (6.20) — — −0.3 ±0.0 Pr Native 55.28 12.95 9.0 (6.0) 7.88 ± 0.42 0.18 ± 0.03 2.82 ± 0.16186 (8.40) — — 17.8 ± 2.2 X-linked 18.3 5.7 — 8.37 ± 0.65 0.38 ± 0.131.41 ± 0.03 130 (7.12) 7.09 ± 0.08 15.1 ± 0.4  5.9 ± 0.1 Xerogel 58.773.84 8.6 3.48 ± 0.04 1.56 ± 0.19 — 296 (3.51) — — 21.2 ± 0.2 Nd Native55.56 10.82 7.0 (6.4) 7.73 ± 0.24 0.19 ± 0.02 3.14 ± 0.41 384 (10.9) — —17.9 ± 1.3 X-linked 20.1 4.2 — 8.71 ± 0.43 0.46 ± 0.06 1.39 ± 0.01  144(14.89) 19.09 ± 4.03  36.4 ± 6.4  4.2 ± 0.2 Xerogel 59.43 3.12 7.4 3.38± 0.01 2.09 ± 0.10 — 307 (3.04) — — 20.3 ± 0.5 Sm Native 56.31 10.42 6.3(5.9) 7.52 ± 0.17 0.22 ± 0.02 2.97 ± 0.12 383 (9.30) — —  5.1 ± 0.4X-linked 18.1 5.2 — 8.45 ± 0.30 0.39 ± 0.05 1.39 ± 0.01 168 (9.83) 13.43± 0.80  25.9 ± 2.6  0.7 ± 0.0 Xerogel 56.73 3.21 6.5 3.41 ± 0.01 2.04 ±0.01 — 257 (2.78) — —  3.9 ± 0.1 Eu Native 56.7 11.31 5.8 (5.2) 7.61 ±0.32 0.20 ± 0.02 2.47 ± 0.14 379 (7.67) — — 16.6 ± 1.5 X-linked 19 2.29— 7.70 ± 0.68 0.53 ± 0.08 1.42 ± 0.03  144 (10.17) 14.34 3.6  5.2 ± 0.2Xerogel 59.22 4.6 6.6 3.20 ± 0.05 2.11 ± 0.05 — 300 (2.78) — — 18.5 ±0.4 Gd Native 58.54 9.63 5.8 (5.1) 8.04 ± 0.36 0.18 ± 0.02 3.14 ± 0.24383 (10.9) — — 90.8 ± 1.3 X-linked 20.6 5.79 — 8.17 ± 0.21 0.44 ± 0.011.40 ± 0.03 171 (9.76) 10.90 ± 2.26  28.5 ± 5.2 30.6 ± 0.3 Xerogel 59.834.14 5.8 3.38 ± 0.02 2.02 ± 0.02 — 294 (2.84) — — 101.6 ± 3.7  Tb Native57.4 8.97 5.2 (4.6) 7.65 ± 0.38 0.20 ± 0.01 3.32 ± 0.31 365 (8.82) — —130.5 ± 5.9  X-linked 19.1 3.9 — 8.30 ± 0.14 0.42 ± 0.03 1.40 ± 0.01 160(9.46) 6.30 ± 0.73 29.6 ± 7.5 43.6 ± 0.4 Xerogel 58.13 3.2 5.7 3.48 ±0.02 1.96 ± 0.02 — — — — 145.3 ± 2.1  Dy Native 59.3 7.42 5.0 (4.1) 8.02± 0.09 0.18 ± 0.01 3.02 ± 0.13 366 (8.23) — — 167.6 ± 3.5  X-linked 214.72 — 8.16 ± 0.06 0.46 ± 0.06 1.44 ± 0.01 176 (7.55) 2.62 ± 0.28 20.5 ±3.2 59.2 ± 3.1 Xerogel 60.21 3.98 4.8 3.44 ± 0.06 2.00 ± 0.12 — 295(2.75) — — 181.7 ± 3.2  Ho Native 57.46 9.63 4.3 7.57 ± 0.18 0.21 ± 0.012.47 ± 0.22  358 (10.92) — — 139.1 ± 7.0  X-linked 20.6 4.28 — 8.35 ±0.20 0.42 ± 0.03 1.41 ± 0.02 177 (14.9) 6.57 ± 1.31 19.3 ± 1.3 54.8 ±3.4 Xerogel 57.36 3.23 4.7 3.39 ± 0.12 2.13 ± 0.04 — 278 (2.94) — —167.5 ± 4.5  Er Native 59.66 7.5 4.9 (4.0) 8.36 ± 0.25 0.16 ± 0.01 3.28± 0.26 368 (8.9)  — — 104.5 ± 5.4  X-linked 22.4 5.34 — 8.41 ± 0.34 0.40± 0.05 1.14 ± 0.01 157 (7.02) 13.65 ± 1.41  16.4 ± 2.8 43.9 ± 0.3Xerogel 59.64 4.8 5.7 3.43 ± 0.02 2.04 ± 0.05 — 299 (2.93) — — 133.2 ±4.2  Tm Native 60.73 6.43 4.8 (4.0) 8.63 ± 0.57 0.14 ± 0.02 3.17 ± 0.12349 (8.88) — — 66.2 ± 1.7 X-linked 21.8 4.75 — 8.59 ± 0.04 0.34 ± 0.011.44 ± 0.01 170 (9.21) 5.65 ± 0.75  9.8 ± 1.4 31.1 ± 1.1 Xerogel 60.323.73 4.3 3.38 ± 0.02 1.96 ± 0.09 — 312 (2.92) — — 84.5 ± 4.2 Yb Native64.29 9.06 4.7 (4.0) 8.43 ± 0.08 0.15 ± 0.01 3.25 ± 0.16 345 (7.89) — —31.2 ± 1.9 X-linked 21.9 1.96 — 8.38 ± 0.12 0.38 ± 0.01 1.49 ± 0.01 — —— 10.9 ± 0.4 Xerogel 61.69 8.18 4.2 3.43 ± 0.03 1.73 ± 0.15 — 323 (3.47)— — 28.1 ± 0.2 Lu Native 64.56 4.03 4.4 — — — 214 (9.64) — —  0.1 ± 0.3X-linked 23.7 3.03 — 8.35 ± 0.07 0.39 ± 0.02 0.39 ± 0.02 120 (7.78) 7.25± 0.30 18.7 ± 5.1  0.9 ± 0.0 Xerogel 59.85 3.02 4.2 3.49 ± 0.03 1.96 ±0.05 — — —  0.08 ± 0.04 ₁Force at rupture and modulus values arereported only for the cross-linked aerogels because the native aerogelswere too fragile to measure these values, and the corresponding“Xerogels” collapsed.

These aerogels were characterized by elemental analysis, infraredspectroscopy, thermogravimetric analysis, and nitrogen adsorptionporosimetry. Properties of interest were chemical composition, bulk andskeletal densities, mesoporous surface area and pore size, mechanicalstrength as well as magnetic properties. Infrared spectroscopy andcarbonate analysis indicated that these metal oxide aerogels reactedsomewhat with carbon dioxide during the supercritical drying processfrom carbon dioxide. Remarkably, however, the incorporation of carbondioxide as carbonate was not quantitative, which would probably have ledto disintegration of the aerogel monoliths. The carbonate incorporationin the aerogels was also evident by the relatively smaller magneticsusceptibility of the aerogels relative to the corresponding “Xerogels,”which were obtained by drying the wet gels under vacuum.

Skeletal densities of native rare earth aerogels are significantlyhigher than the skeletal density of silica, reflecting their higheratomic mass. However, bulk densities of all rare earth aerogels arecomparable to one another and in the same range as the density oftypical silica aerogels, reflecting the small amount of solids in theceramic oxide skeletal framework. Surface area analysis yields highsurface areas for the native aerogels and somewhat decreased values forthe corresponding cross-linked aerogels, similarly as for silica. Againthe most notable property enhancement for the cross-linked aerogelscompared to their native analogs was increased mechanical strength.

Microscopically, all metal oxide aerogels look very similar to oneanother and very similar to typical silica aerogels. Representativescanning electron micrographs of selected native metal oxide aerogelsamples are shown in FIG. 10, where it is seen that typical nativeaerogels have a similar particulate network structure, comprising arandom distribution of a pearl necklace-type strands of nanoparticles,consistent with the structure of silica discussed above. It was alsofound (with one exception) that the mechanical properties of non-silicacross-linked aerogels were not much different from one another or fromcomparable cross-linked silica aerogels of similar density. VO_(X)aerogels, based on vanadium, appear from SEM micrographs to look asthough primary particles form secondary worm-like structures rather thanan agglomeration of mostly round secondary particles. See, e.g, FIGS. 11a and 11 b, showing the microstructure of a native and a diisocyanatecross-linked vanadium aerogel, respectively. Both these samples had thesame bulk density for the uncross-linked VO_(X) aerogel (prior tocross-linking in FIG. 11 b). The two samples appear very similar, eventhough the density of the latter is four to six times greater than thatof the former (native VO_(X)), indicating rather heavy polymer loading.Their apparent similarity indicates that the isocyanate cross-linkingpolymer structure (polyurethane/polyurea) was deposited as asubstantially conformal coating over the mesoporous secondary surfacesof the VO_(x) aerogel.

Cross-linked vanadium aerogels proved to be tough materials relative toother types of crosslinked aerogels. FIG. 12 compares the stress-straincurve of a typical Desmodur N3200 diisocyanate cross-linked vanadiaaerogel having a bulk density of 0.38 g/cc under compression testing,with the analogous curve of a Desmondur N3200 diisocyanate cross-linkedsilica aerogel of similar density (0.48 g/cc) under the same conditions.Compression tests were conducted according to ASTM D695-02a using ˜½inch diameter by ˜1 inch long monolith samples of the respectiveaerogels. It is noted the cross-linked silica sample shattered at 77%strain under ˜25,000-30,000 psi stress, while the cross-linked vanadiumaerogel remained in one piece (although it was nearly fullyconsolidated) even after more than 100,000 psi stress and more than 93%strain.

It is highly probable that the morphology of the underlying skeletalnetwork of cross-linked aerogels coupled with the extremely high polymerloading plays a significant role in the added toughness. In that regard,fibrous structures such as those evident of vanadium aerogels (see FIGS.11 a-11 b) are probably more apt to retain some structural integritythan the more globular pearl-necklace type microstructures. It isexpected that this realization will comprise a point of departure forthe design of a plethora of lightweight materials with different failuremodes based on the general concept of a polymer coated three-dimensionalassemblage or network of nanoparticles.

As noted above and demonstrated in examples, cross-linking the solidceramic oxide network in a wet gel results in the corresponding aerogelhaving superior mechanical strength compared to the native ceramic oxideaerogels. In addition, such superior strength, quite surprisingly andunexpectedly, has been found to come at only a modest increase inoverall or bulk density for the aerogels. Without wishing to be bound bya particular theory, it is believed that the dramatically improvedstrength qualities of the polymer cross-linked aerogels disclosed hereinmay be attributable to one or more of the following reasons.

The mesoporous structure of conventional aerogels, (which is largelyresponsible for their desirable properties such as high specific surfacearea, low density, low thermal conductivity, etc.), results from thesol-gel process for producing the corresponding wet gels via hydrolysisfrom an appropriate ceramic oxide precursor, e.g. TMOS for silica,followed by supercritical fluid extraction to produce the structurallyintact and dried aerogel. Physically, the mesoporous structure consistsof fully dense 1-2 nm amorphous particles, which in many ceramic oxidesassemble or are agglomerated into ball-like secondary particles (5-10nm). These secondary particles are connected together at interfacesknown as neck regions located between adjacent secondary particles,fashioned by dissolution and re-precipitation of the sol gel duringaging. This structure also is apparent for several of the polymercross-linked silica aerogels described herein, based on theirmicrographs shown in FIGS. 4 and 8. These voids located betweenentangled and interconnected nano- (secondary) particle strands comprisethe aerogel's mesoporous network that is responsible for the desirableproperties noted above. An exaggerated neck region between two adjacentsecondary silica particles is illustrated schematically in FIG. 9, whichalso illustrates a reaction for cross-linking the particles viapolyurealpolyurethane chains extending between respective surfacehydroxyls on the particles.

When stress incident on a conventional aerogel causes fracture,fracturing is believed to occur primarily at the interfaces of secondaryparticles (necks), while secondary particles themselves remain intact.The incorporation of a nanocast polymer coating covalently bonded to thesurface of the ceramic oxide framework before drying to produce theaerogel is believed to reinforce these neck regions by widening them sothat they are better able to withstand mechanical loads. At the sametime, because the polymeric networks are provided in the form ofsubstantially conformal coatings over the interconnected particle strandnetwork and do not extend substantially into the mesopores definedbetween the strands, the polymeric networks are able to provide suchreinforcement while only minimally reducing porosity and increasing bulkdensity. This would explain the observed 2 to 3 times increase inmeasured bulk density for cross-linked aerogels, but 100-200 or moretimes increase in physical strength properties such as elastic modulusand ultimate strength (stress at break).

Another benefit of the polymer cross-linked aerogels disclosed herein isthat they are much less hygroscopic than native aerogels, which has beenanother problem inhibiting their use in many applications. The observeddecrease in hydrophilicity is believed to be due to the reduced numberof Si—OH groups on the surface and the addition of the largelyhydrocarbon conformal coatings from the organic cross-links. Indeed, useof fluorocarbon containing monomers such as pentafluorostyrene as anadditional cross-linker in the styrene cross-linked aerogels reduceshydrophilicity even more.

Although the hereinabove described embodiments of the inventionconstitute the preferred embodiments, it should be understood thatmodifications can be made thereto without departing from the scope ofthe invention as set forth in the appended claims.

1. A structure comprising a solid-phase three-dimensional network ofceramic oxide particles having functional groups bound to surfaces ofsaid particles, said network of ceramic oxide particles beingcross-linked via organic polymer chains that are attached to saidparticles via reaction with at least a portion of said surface-boundfunctional groups, said ceramic oxide having the form ZO_(x) where ‘Z’is a metallic or semimetallic element other than silicon.
 2. A structureaccording to claim 1, said surface-bound functional groups comprisinghydroxyl groups native to ceramic oxides.
 3. A structure according toclaim 2, said surface-bound functional groups further comprisingnon-hydroxyl functional groups.
 4. A structure according to claim 1,said polymer chains being polyurea chains extending from or between, andlinked to, surfaces of said ceramic oxide particles via carbamatelinkages formed through reaction between a surface-bound hydroxyl groupand an isocyanate group on an organic polymer or polymerizable species.5. A structure according to claim 4, said metallic or semimetallicelement being an element selected from the group consisting of: Sc, Y,La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and V.
 6. A structureaccording to claim 1, wherein ‘Z’ is a mixture of metallic orsemimetallic elements other than silicon, such that said ceramic oxideis a composite ceramic oxide composed of a plurality of species ofmetallic and/or semimetallic atoms linked via interposed oxygen atoms.7. A structure according to claim 1, said three-dimensional network ofceramic oxide particles being in the form of a dried aerogel that iscross-linked via said organic polymer chains, said polymer cross-linkedaerogel having a bulk density not more than about 10 times greater thanthe bulk density of the corresponding uncross-linked aerogel, but atleast 10 times greater elastic modulus than the correspondinguncross-linked aerogel.
 8. A structure according to claim 7, saidpolymer cross-linked aerogel having an elastic modulus that is at least100 times greater than the elastic modulus of the correspondinguncross-linked aerogel.
 9. A structure according to claim 8, saidpolymer cross-linked aerogel having a bulk density not more than about 3times greater than the bulk density of the corresponding uncross-linkedaerogel.